Mustard Algae

Mustard Algae In Pool - How To Get Rid Of The Yellow Menace?

Mustard algae, sometimes called yellow-green algae, is very distinct from other types of algae. Aside from its peculiar colour, it is also not very effective at attaching itself to surfaces. For this reason, mustard algae often gets mistaken for other types of pool pollution.

In this article, we will cover how to identify mustard algae and the most common sources of it. We also include a step-by-step guide for removing mustard algae from your pool, and our top tips for helping you prevent it from developing again in the future.

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How to Identify Mustard Algae

Mustard algae is most easily identified by its unique colour and texture. This algae presents in shades of yellow, giving it its colloquial name. Unlike other types of algae, mustard algae does not have a slimy texture, but tends to appear more as a powder or dust.

Many pool owners mistake mustard algae for dirt, sand, or pollen due to the texture and the way the algae behaves. If you lightly rub your hand on a patch of mustard algae, it will easily come off and begin to cloud the water.

This type of algae can be difficult to treat because its cells are especially small and tend to be chlorine resistant. This means a mustard algae infestation can develop even if your pool has had adequately maintained chlorine levels.

Causes of Mustard Algae

Mustard algae is a common type of algae in warmer climates, and therefore it easily flourishes in freshwater lakes and streams. The most common cause of mustard algae in your pool is cross-contamination that occurs after swimming or using pool toys in these natural bodies of water.

Mustard algae has unique properties that allow it to become completely dried out while still remaining viable. Once dry mustard algae is introduced to your pool water, its cells can rehydrate and begin a vicious reproductive cycle.

Mustard algae can also be introduced to pools by using pool equipment that has not been properly cleaned after a prior algae infestation. This is common with second-hand pool tools such as brushes, nets, telescopic poles, and vacuum heads.

How to Remove Mustard Algae From Your Pool

The first step you will want to take to remove mustard algae from your pool is to scrub the walls and surfaces of your pool. This does not take as much elbow grease as it does with other strains of algae, but should still be done thoroughly.

After this, you will want to vacuum your pool to waste. If you want to combine these first two steps, you may opt to use a vacuum head with a brush attachment.

Next, it is necessary to shock your pool. You may choose to use a chlorine-based or non-chlorine based shock treatment, but it is important to keep in mind that mustard algae can be resistant to chlorine. Therefore, if you choose to use a chlorine-based shock treatment, a double or triple dose is strongly encouraged.

Use this calculator to work out the appropriate dose for standard shocking.

Because mustard algae cells can survive even when dried out, we recommend throwing any pool equipment that has been in contact with infected water into the pool before introducing the shock treatment.

This is not a remedy for all infected pool toys and swimming equipment, and may not be appropriate for certain materials, but it is essential to properly clean your tools and anything else that was in the contaminated water, such as swimsuits.

Prepare your pool’s chemistry for the shock treatment by following the instructions provided by the product manufacturer, then shock your pool. After the shock treatment, you will want to clean and rinse your filter thoroughly in order to remove as much algae buildup as possible.

When your chlorine levels return to normal, you will want to use an algaecide. This should typically occur at least two days after a chlorine-based shock treatment, but this waiting period may not be applicable with a non-chlorine based shock.

For an algaecide, we recommend Lo-Chlor Algaway, which is designed to be effective on mustard algae and can work to prevent new growth for up to three months. Regardless of the product you choose, always follow the manufacturer-provided product instructions for adjusting your pool’s chemistry in order to prevent risky, unwanted chemical interactions.

At this point, if you have noticed algae reaccumulating on the surfaces or bottom of your pool, it may be a good idea to repeat brushing and vacuuming. Although mustard algae does not attach itself very firmly, having large accumulations of it that are not fully exposed to the treated water can allow hundreds of cells to survive and eventually reproduce in the thousands and millions.

Two days after adding your algaecide, it is recommended that you perform another shock treatment, this time using a regular dosage. This second shock treatment is intended to kill any leftover algae cells that may be remaining, and further reduces the likelihood of a recurring infestation.

One final filter cleaning is recommended as you wrap up your algae treatment process. This not only reduces the chances of surviving algae cells in your filter, but also allows for your filter to remove larger quantities of dead algae at a faster rate.

If you notice staining of your pool walls after the mustard algae are gone, check out this guide on how to remove this staining.

Preventing Mustard Algae in the Future

The best way to prevent mustard algae in your pool is to avoid cross-contamination with natural waters that occurs via swimsuits, pool toys, and other related equipment.

If you enjoy swimming in or floating on natural waters regularly, it is a good idea for you and your family to have swimsuits, towels, and toys specifically for this purpose that do not come in contact with your swimming pool’s water.

This reduces the need for the frequent washing of swimming garments while making sure that your pool stays free of any potential contaminants.

Because mustard algae is resistant to chlorine, maintaining adequate levels of it is not likely to be an effective preventative measure.

Instead, we recommend the continued use of small amounts of algaecide as a preventative product. It can also be helpful to test your pool water for high levels of phosphates, as this may be feeding the algae.


Because mustard algae has a somewhat strange behaviour and appearance, it can come as a surprise to many pool owners. In reality, occurrences of mustard algae in swimming pools are not that rare among warmer climates.

Once you have taken diligent steps to treat your mustard algae and have learned ways that you can prevent it from occurring again, the yellow menace becomes much less intimidating.

Do you have any questions about removing yellow mustard algae from your pool? Leave us a comment down below!

Louis from Pool Advisor


A chemical engineer by trade, Louis is committed to debunking myths in the pool industry by explaining the underlying chemistry and making it accessible to all.