Usually its a sign of low pH, low KH, low flow or high organics. Or any combination in between.

Have you water tested. Chemi-clean is an excellent product but if your chemistry is off, the slime will be back. Cyanobacteria is a single celled colonial photosynthetic bacteria that clumps in low flow areas. It likes water with high CO2 and low pH. So change that and it tends to go away on its own. And if it doesn't, hit it with Chemi-clean, turn off your skimmer, wait 48 hours and do a 20% water change. Usually very effective.

Also, Cyano itself is not detrimental, but it usually indicateds something is off.