"Crying?"

Strange (or the body formerly known as Strange, whatever) properly covered for now, Childermass is free to actually look up in mild surprise. While he doesn't think it as impossible that the Ringmaster could cry, he can't fathom what Lambert could have done to cause it. And yes, he is assuming the cause is Lambert. It makes the most sense, knowing the witcher well enough.

"What has he done now..."

It's less a question, more a statement.