reefhead76

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 and can no longer be removed with a turkey baster this piece sits high on my tank but not directly under a light. about 6 months ago i moved him to the bottm of the tank but the necrosis continued. I have now adjusted the light to the left and put him to the right in the shadow of an overflow.

All water params check out. Sg1.025, phos 5, nitrates 0, kh 9, any thoughts or experience with this issue is appreciated.