I have a brain coral that had a similar, albiet small white area when I purchased it. The LFS said it had been too close to another coral and had been stung in that area. Since putting it in, it has very slowly started to heal in that area. (The healthy new tissue coming from neighboring cells and spreading.) It does like alternating moderate flow. And even these corals seem to produce some "mucous" once in a while.
Good luck!