Mick didn't know who this fish man was, or what he was really, but he had Beth and saving her was the first thing to do. "You heard her, names Mick, and I told you to let her go."
His eyes hadn't changed and his teeth hadn't elongated yet, but if he still refused to cooperate, then Mick would have to resort to his vampire self. If she were claiming that she didn't know anyone named William Turner, then Mick believed her.