coral dying?

[MitchM]

Hard to tell from the pic, but if that's a sarcophyton or leather coral, as long as your water parameters are good, it is normal behaviour for that type of coral. It sometimes will shrink and shed a protective mucous layer.
Make sure that you have alternating water flow around it and all other corals.
That type of coral releases toxins that are harmful to other corals.
Have some activated carbon running to help absorb those toxins.

Examine the coral to see if you can see any areas that look rotten or are dissolving away. In that case, the coral is sick and you may need to cut off a healthy area in order to regrow it elsewhere.

Mitch