Dusting of cyanobacteria. Increase flow in the tank as it doesn't grow in higher flow areas. It feeds off of nutrients in the water column, but since there is always some in there it doesn't always indicate a parameter issue.
I didn't say reduce feeding. You might be feeding fine. It's the flow that should be adjusted. Just because it is there doesn't mean it is a problem. Cyano in small amounts is flow. If you had loads, we'd be talking other things.