Well, the first question is, "How have you been treating your pool, in the past?"
'Watermold' is more common in pools with Baquacil, etc than in chlorinated pools.
PS. If it *is* a chlorinated pool, all you need is chlorine . . . and a good testkit. But I don't want to give you advice, assuming you have a chlorinated pool, and then end up staining the heck out of things because you've been using 'minerals' or else starting a Baquacil conversion without meaning to do so