Mustard algae, also called yellow algae, is a type of algae that can form in swimming pools. While it is not as common as other types of algae, it can be difficult to get rid of once it forms. Mustard algae typically form in areas of the pool that are poorly circulated, such as in the corners or around steps. It is important to treat mustard algae as soon as possible, as it can spread quickly and make the pool difficult to use.