Susan doesn't know what's happened, but he can still see and feel the tension, even if there's no immediate fighting happening. He looks between Childermass and the merman, frowning as he tries to figure out just what's going on. If he'd been here earlier he might know better, but as it is... He only has to take his word for it, combined with the fact that he generally assumes the best of everyone. A misunderstanding at least seems like a possibility, who knows what mermaid culture is like.

He nods a little, still ready to try and fight if it really is a fight, but otherwise relaxing.

"Well, if you're sure... Misunderstandings do happen sometimes."