Do you make RO or RO/DI? If your RO has any TDS it will just feed the algae when you do water changes. I agree with remove as much as possible by hand or turkey baster. Then change water with RO/DI no TDS. May have to buy it from LFS. I did that for awhile before I bought a 5 stage RO/DI unit from Bulk Reef Supply. I had a vacation mishap where a huge amount of food fell in my tank. I had Cyano everywhere. GFO, water changes RO/DI, and hand removal and it was all gone in 2 weeks. What type of filter do you run? Do you have a refugium?
I do have a 5 stage RO/DI unit guess I didn't say that. I have a 36" sump with a refugium. And I run a reef octopus skimmer. I will be upgrading everything when I get the 210 going.