Looks like the tissue is dying back. Possibly due to an infection, parasite, or a critter in eating it...
First thing I would do is frag off some of the healthy tissues. The more frags you can make the better, that will give you some 'backup' if the main colony dies back.
Second, post up your water parameters, including ammonia, nitrates, phosphates, ph, salinity, calcium, mag and alk. The more information you can provide the better.
Also what else in the tank, a full tank shot could be useful as well as all inverts and fish you have in there.
If it is an infection or parasite, you are going to need a hospital tank for treatment, so it would be a good idea to get a 10 gallon ready to be fired up...