I agree with DvSkiN. It looks like it's bleaching.
Is the tissue "peeling" off in the middle of the coral? If so, this may be Rapid Tissue Necrosis (RTN). It's unfortunatly common in wild caught Small Polyp Scleractinians (SPS), and often times cannot be stopped. RTN is usually identified when the tissue of the coral is actually peeling off (Normal bleaching usually is when the tissue gets lighter and lighter until the calcareous skeleton is showing bear). If it is in fact Rapid Tissue Necrosis, you would be best off breaking off the infected areas of the coral, leaving only the un-infected areas in the tank. The infected areas with RTN can be removed from your aquarium, so it doesn't infect other Scleractinian corals.
Graham