Corals can stress for a variety of reasons and become susceptible to a variety of bacterial agents.
About the only way to save it now (because I suspect it has a brown jelly infection) is to give it a series of saltwater dips with either Lugol's or Seachem's Reefdip to try to stop it, and perhaps even a double strength dip.
Here's how:
You could take some tank water out of the main display and in one separate bucket, take a turkey baster and GENTLY blow off the excess/dying tissue from the skeleton. Then in another SEPARATE container, I would have an aerated bath waiting for the coral and let it sit for about 15-20 mins in the medicated dip. Read the directions for amount. Then have another small amount of tank water ready to gently wash away the iodine dip water.
If the bacterial infection seems really rampant, you could take a small Q-tip and gently swab the main infected area of the coral with straight iodine prior to the medicated bath; rinse with non-medicated tank water after the bath; and return to either a QT tank or the main display. Give the coral moderate flow for now (enough to keep the sloughing off).
Try not to blast the coral with the powerhead inside the main display tank, as this infection can be contagious to other corals. If it's too late for that, you might consider siphoning off any loose tissue you see in floating around in the tank.
And I would not attempt to feed the coral until the bacterial infection is under control. Sometimes the excess food can just "feed" the bacterial growth and make matters worse.
Also, if the infection is really bad, you might result to 2 dips/day (am/pm) to try to get it under control. Keep in mind this is a very swift infection and the whole colony could be wiped out in a couple of days. Act now if you want to attempt to save it!
Good luck and keep us updated
JAM
