It's cyanobacteria. It's possible that it's smothering your polyps. Try syphoning it off using some airline tubing or, use a baster to blow it off. You could also try aiming a ph
toward the polyps. That should help the cyano
from taking hold.
As for the cyano
itself, it's caused by excess nutrients in your water. High nitrates and phosphates will aid it's growth and low flow helps it attach.