I've not heard of this with tangs, but it doesn't surprise me too much. They have small rasping teeth, just right for going after those type of corals if they wanted too. Think how they can devour coraline algae.
This looks like a case where you will need to choose one or the other the corals or the tang. I'd trade in the chocolate tank on another species myself, but I wouldn't call keeping the tang the wrong thing to do.