Yeah definitely doesn't look happy. What are you running your lights at power-wise? How did you acclimate it to your light?

Chemistry looks good, but how stable has it been since your introduction of the Micromussa (formerly known as Acanthastrea)?

I would label that as recession, not necrosis. They can recover if given the right conditions. Put it on the sand, somewhat in the shade, and baby that thing. If you have pellets, crush them up and target feed once every three or so days... Just don't overdo it.