Agreed this is tissue ressesion due to something like not enough light or flow or something was off in chemistry. Correct the failing condition and it will come back in time.
Well nitrates were 1 and phosphates were at .05 last night. Ph at 8.2, temp is consistent between 78-78.9, alkalinity roughly 8.7-9, calcium 420-450, magnesium around 1460-1500, which I don't know how because I don't dose it. And Red Sea coral pro salt isn't that high.
My apex par meter on bottom of 40 gallon breeder gets a par reading of roughly 110-116. And it's sits about half way up. I have a mp40 on opposite side running around 20-25% for about 16-18 hours. So that's about 800 gph
Also hermit crabs go in there and hang out. Could they me irritating it? Should I feed this at Day, because at night when I feed my other corals the side, whatever they're called are inside the coral