This one probably doesn't happen very often but I thought I'd throw in here just for the fun. I had a green brain coral that I'd had for about 2 years. It was doing awesome. Couldn't have asked for a happier brain. Then one day it shrunk in and got smaller and smaller and wouldn't come out again. Tested levels and everything was perfect. All my other corals where thriving. They all where getting bigger while the brain was dying. I picked the brain up and noticed a little hole on the back side of his skeleton. Couldn't see anything abnormal in the hole so I positioned the brain in another part of the tank, hoping that maybe it just didn't like its spot it was in. Then the next day I saw something jump down the hole. Further investigation found it to be a small barnacle. Atleast I think it was a barnacle that had created the hole and was eating the brain from the inside. So I pulled the brain out and performed my first brain surgery.
I took a pair of tweezers and piece by piece ripped the foreigner out, treated with Iodine and put the brain back in the tank, but it was too late. The brain died.
So long story short is check for unwanted hitchhikers.