Mustard algae is often mistaken for dirt or sand on the sides of the pool. Mustard algae is resistant to chlorine and is a form of green algae which presents as yellow-green or brown in color. This type of algae is found in areas where there is constant sun on pools during the day, and is not as common in the northern climates where it is often mistakenly identified as the source of the issue.

When determining if you have mustard algae or dirt, if it feels gritty it is dirt or could be calcium scale. If it feels slimy, it’s probably mustard algae.

Mustard algae can be brushed away easily (calcium scale will not brush away), though it will return quickly to the same location. Algae may form in areas in the pool where there are dead spots or dead zones, meaning the water is not circulating in that area. Mustard algae will also climb pool walls. Hint: dirt will not climb walls, and will remain settled on the floor. Though calcium and mineral scale can form on the walls.

Mustard algae will increase the demand for chlorine in the pool, though it may survive even with shocking. It is very important when treating mustard algae to treat and clean all equipment including ladders and inside light niches, as well as any pool equipment, cleaners, or toys and even bathing suits that have been exposed to mustard algae, as it can recontaminate the pool. Pool equipment can be treated by leaving it in the pool during the treatment with chemicals, or cleaned with a mild cleaning solution.

To treat yellow algae, have the water tested to determine the exact water chemistry levels. Brush the pool thoroughly before, during and after treatment so the algae slime will be opened and exposed to the chemicals treating it. Aggressively shock with chlorine, and follow your algaecide directions.

In the case of a Baquacil Pool or pools with alternative sanitizing methods such as Soft Swim or Buguanizes, the Bactericide level should be topped up, and double doses of shock and algaecide should be used.