I have not seen this before on a sun coral. If its tissue of the coral it might be that something decided to try and eat it. Have you taken the coral out and dipped it? Just to check and see if this stuff detaches or see if something else comes off as it could also be a parasite of some kind. How long have you had the coral? Looks like it is eating well from the picture which might also attract some attention from other tank inhabitants that could damage the coral looking for food the coral consumed. I am curious to see if anyone else has seen this before.