Sounds like algae to me. My father turned me on to BioGuard, made by Valvtect. He uses it in his trawler with great success. It's easy to get and only takes 1/2oz per tankful after an initial 1oz treatment. Not only does it kill algae and bacteria, but it also renders them able to be burned up and clears them out. See link below.
BioGuard
