Is it getting stringy? It is likely diatoms, which are a result of excessive amounts of silcates and phosphates. I just went through. Spell of brown diatoms, and am now dealing with a green version of diatoms that I mistakenly thought was algae.
The source of diatoms? Tap water! Are you purchasing RO
water, or using your own personal RO
system? If not, it will be hard to keep these diatoms away! I learned the hard way ... hopefully this helps those of you who were in my shoes a few months ago.