Aziraphale just keeps his mouth shut, and if he's smiling, it's only because he's excited to be entertaining others.

Fortunately (or unfortunately for Moloch), the angel's magic tricks go off without a hitch, and no one else seems inclined to complain. Aziraphale himself is very warm and comfortable to curl up against, so at least Moloch won't feel squished or otherwise inconvenienced while he waits for his chance to strike.