Ailing open brain coral

[SeaShel]

I have a fairly small (about 3" in diameter) brain coral that has been doing well in my tank until recently.
There have been no other changes (parameter or otherwise) in my tank since its introduction.
Recently it does not appear to be opening very far and I have noticed some discoloration along one edge. I also this morning saw one spine of skeleton showing on the underside of it.

It eats when fed (about twice a week) still, including yesterday.

I will be out of town for four days starting Sunday and do not feel confident leaving my husband to know what to do with it.

Any ideas on what may have gone wrong would be appreciated.

Thanks in advance.

[JENnKerry]

Where in the tank is it located? What are your water parameters? How much and what kind of lighting? Tank inhabitants?
Sorry for all the questions but it will help us understand a little better.

[SeaShel]

Parameters are as follows but haven't changed in the three months I've had it:
Temp 80
Calcium 385
Alkalinity 2.8
pH 8.2
No ammonia or nitrite
Nitrate 5

Tank is 72 gallon reef with metal halides and actinics

Brain is on bottom sand bed with nothing in its reach

Fish and shrimp do not bother it. Clown has a different home. Shrimp are disinterested except when I feed.

Thanks for any help you can give. I'd hate to see it die.

I have moved it today to an area with more shade.

[djtodd]

Chemical warfare? I have a folded brain that started doing poorly after a tank migration. Started running Carbon again and it got all better within a day.