i had a large purple tang years ago that would pick at my welisophylia , he would make his rounds picking about the tank in a particular routine, simply moving your scolly to a different location in your tank may keep your fish from picking at it , also make your he is getting enough to eat an algae clip and some nori is something i try to keep my tangs full through the day if there is not enough for him to graze on in your tank , another thing that can be done to break up his routine is the cut the top of a 1 or 2 liter coke bottle off and put it over your coral drilling small holes in it and leaving the cap off to allow water flow thru the bottle half .... also works great to spot feed the coral when fish , crabs and other tank mates from stealing its food away.