It probably just got startled and went bang into your live rock. As the others have said, it will heal. Just keep an eye out for secondary infections etc.
On the other hand, my female clown has a lump under it's chin. When the clown is really stressed out the lump can get reddish and it takes a long time for it to go away.
I hope your yellow tang doesn't have something like this.
An aside, it occurred to me the other day when I was looking through an aquarium book that had close up photos of corals and fish in a natural reef environment, just how many little deformities and abnormalities etc that natural reef inhabitants have. For example there were some sps that had patches of dead bits here and there. Just made me feel more grateful for the fact that most of my fish are near perfect. Well actually, on closer inspection they aren't. In fact they are far from perfect! My female clown has a recurring lump on it's chin and my bloody coral beauty hasn't had a tail for about three years (imagine explaining that to visitors).
My yellow tang is rare perfection.