Credence doesn't pick up on it--it's probably incredibly apparent, but her excitement causes him to withdraw, not because he's scared but because he wants her to be as excited as she is and he doesn't want to intrude.

And, of course, he's not looking her in the eye. He's glanced up enough to notice how entrancingly green they are, but beyond that, he stays in his own personal space.

"I thought it was a woman in a lake," he says softly, and that's when he looks up again, perplexed. "I know it, but--they're old fairy tales. Arthur, and the sword in the stone, and Merlin."