Hmm well usually damsels are the ones that pick on others, not the ones who get attacked, but thats what it looks like. Might have gotten chased and ran into some rocks? If its a one time injury, just keep him well fed and he should start looking better in like 2 days. If he starts looking worse, Id be more concerned and think about isolating him untill you can figure out if its agression, an ailement, or some possible water chemistry problem thats causing a lot of stress.
If you seperate him and he gets better, probably agression.
If you seperate him and he dies, but the rest of the tank goes on fine, probably an infection that he got somehow.
If you seperate him and the fish in your display decline with his absence, then you have someonething bad in your water, either living or chemical.