Judging from the wound, I'd say your other tang had a disagreement with your convict tang as to who was going to be boss fish.
Usually these wounds heal quickly, so don't do anything unless you see it getting worse.
In my opinion, using any type of conditional with a slime coat is a big mistake in a reef system. This sort of glop can interfere with all sorts of other livestock and bacteria in the tank. In a quarantine tank, where it's just the fish, then maybe, since there is no other livestock and little bacteria.