I had a REALLY BAD case in my 20g reef for the past few months. It was 'growing' and multiplying like hair algae and a disease. I tried Chemi-clean that usually works for me, and water changes, but these didn't work this time. I think this was too strong, and also immune to the chemi-clean or something. It was so bad I was taking each rock out and cleaning it off, even putting in new rocks and they would get covered in a day, and the cyano was even growing straight up in strands into the water column, over all the snails, corals, in 3-4 inch strands all over the glass (yes, cyano, seriously)...well, more like a mix of cyano and dinoflagellates, I think. But I noticed that any shaded parts were not overgrown. So I took out most all the rock except one piece, cleaned off what I could, and turned out the lights and kept them off for a couple weeks. You may not have this option, as this was in a little tank of mine, and I transferred any corals I had in there (only a few) into my larger 75g tank. But this method did work. There is NO trace of it at all, when this was the worst and grossest case I'd ever seen. Even if you can't leave lights off 24/7, you could seriously reduce your photoperiod and see if that helps.
