I would do something about it because if Mikey did something to the other dog (in defense obviously), then it wouldn't matter that the other dog was aggressive, they would probably place the blame on him. We had an aggressive dog once (ages ago) and the situation was not pretty when he ended up biting someone. So, I think it'd be best to call AC and see if there is anything they can do. You don't want Mikey to get hurt or the little ankle biter to end up bleeding.