Propagation - fish & corals

I purchased a nice Aussie platygyra which i suspect may actually be a Goniastrea I picked up about a year and a half ago. At about 6 months On the very top however the skeleton is taking on a bit of a brown or charred color.

It sort of resembles slime that didn't wash away, turned brown and stayed there but it won't blow off, I recently noticed it spread ...