Make absolutely sure it isn't red slime. Right before I took out my crushed coral and added sand, I noticed some red algae on the crushed coral in the back corner of my tank and on plastic cave thing that I have stuck on the back of the tank. (I would take the plastic thing out, but my tang and coral beauty live in it!) Anyway, when I started fooling with it, it rubbed off pretty easy with my finger. I took the crushed coral out and scrubbed that plastic thing in distilled water. So far so good. Coralline shouldn't be growing on your corals, I'm pretty sure. However, red slime will overtake and eventually smother them. Please make sure.