Like Dr. Marco said, excess nutrients and light cause cyano, as well as parts with no flow. I battled red slime for a long time, until i got some more flow in the tank, and i saw it decrease for a bit. Now i just have cyano growing in a rock, just because there's not too much flow going there due to my corals. A nice addition would be to pick up a Phosban reactor, since that helps lower phosphates, which is the main reason why cyano or red slime occurs!