User blog comment:Merlaa/Do you think Morgana knows?/@comment-4650675-20121222230702

It wouldn't surprise me. Then again she had never heard of the name Emrys before the Cailleach told it to her...perhaps she saw it in a vision or something?

Or maybe her expression was just satisfaction. You could tell throughout the episode that she was very happy to have Mordred at her side again.