it has RTN, or rapid tissue necrosis. This is common in finicky sps corals. Basically its tissue leaving the skeleton of the coral in a quick amount of time. If you say it had grown before it started to RTN, have you changed anything recently? When a sps corals stressed out, it will RTN. So most likely, the coral does not like where it is now, and wants a better spot.
What are the conditions the coral is in now? Please give us a list or parameters. Is it up high, low in the tank? How much flow is it receiving? A coral that has RTN can be saved, but it will take a long time of having the right conditions.
