Mustard algae is caused by inadequate free and available chlorine levels, a pH level that is too high, or insufficient cleaning or brushing of pool surfaces. Each of these problems allows the tiny, microscopic plants to enter a pool through rain, wind or fill water to establish a home.
Increasing the amount of chlorine in a pool is somewhat effective at preventing mustard algae, but it has a high level of chlorine resistance. The pH level of the pool should be kept between 7.4 and 7.6, and its alkalinity should range from 120 and 150 ppm to inhibit mustard algae growth effectively.
Mustard algae has a powdery consistency that clings to pool walls, and it looks like a yellow stain or sand. Scrubbing the walls and vacuuming out the algae using the waste setting is the first step in cleansing the pool. Shock should be added to remove any remaining algae, and repeated brushing of the walls and floor is necessary over the next several days as the shock does its work. Cleansing any toys, equipment, bathing suits and other items that have been in the pool is essential to prevent reintroduction of the algae once an infestation has been removed.
Mustard algae often appears on the shaded side of the pool. Green algae is more common, free-floating and turns the water green. Black algae forms a slick, black coating on pool surfaces.