I have read that most people do not "feed" thier brain corals. When in reality they do eat. At night when they close up they have tentacles that come out for feeding. It said in the article that while most brain corals will "survive" without feeding that they won't really grow with out it.
It said to wait until after the lights go out and take a turkey baster with some small mysid, and (or) fortified brine shrimp and dust the top of the coral lightly and this should trigger the feeding response. Hopefully this will work for you and maybe get your coral back to normal again.
If I can find the article again I will post a link to it.
Mike