Mustard algae is a type of algae that is yellowish-green in color and often appears in swimming pools. Unlike common green algae, mustard algae tends to cling to pool walls, floors, and other surfaces, making it more difficult to remove. It can also resemble dirt or sand, which can make it harder to identify at first.

Mustard algae is resistant to chlorine, so even pools with proper chemical balance can develop an infestation. It can spread easily through pool equipment, swimsuits, and toys, so it’s important to thoroughly clean anything that comes into contact with the pool.

To treat mustard algae, you typically need to use a higher dose of algaecide and shock the pool with a large amount of chlorine. Brushing the pool surfaces and vacuuming the algae to waste can also help in the removal process. Ensuring proper filtration and circulation in the pool is crucial to prevent its return.