In 'Bereft', as M'gann is viewing her memories again we get a short shot of a white creature in a cave. It does not appear to be anything they have faced before and looks like it could possibly be a White Martian. It also begs the question: is M'gann really a green Martian? Her denial in "Targets" seemed very suspicious to me and Cartoon Network has stated that she's hiding her appearance.
Link to the image: http://imageshack.us/photo/my-images/135/1300239814045.png/ Does it look like a White Martian to you?