Have you ever checked your PO4
is one of the biggest "feeders" of algae. Does your RO
have a DI
stage? The DI
stage removes most of the PO4
Also how much do you feed and how often? Do you drain the packaging juice from the frozen food before adding?
How long is you lighting period every day? Do you have a skimmer?
Its most likely diatoms and they should go away eventually, but nutrient control is the key to elimination.