As the weather warms up and the swim season starts, pool water quality begins to suffer and algae often start to grow. Some algae are easy to treat and remove; however, other types can be quite challenging to deal with. The three main types are: green, black, and yellow/mustard algae. Green algae is relatively easy to eradicate, but yellow/mustard and black algae are difficult to kill and, as a result, can make the job of a pool service professional difficult.