LionFish says.........
Although this is not a common happening, it occasionally occurs with Yellows, Purples, and even Hippos. But like I said very few do it. Your first choice is to give it a varied diet. If you don't feed it a lot of greens (algea, lettuce, seaweed selects, etc.) you should start immediatly. The first way a fish shows it's natural hunger is by eating something that might compensate for it. Your coral has some nutritional value to the tang so it will therefore feed on it until you give it a full and proper diet. Now, if you already do give it a varied diet the only other thing is to get rid of the coral or the tang. Unfortunately it has to come down to that. But either way the tang will eventually destroy it. It depends on which one you like more. I would recommend trading in the tang you have now for a different one. It will save you time and money. Hope this helps a bit.