"Wait, YOU'RE an alien?" Wow does M'gann ever do a double take at that, giving her a good wide eyed stare as she looks over her again. "But...But you look so...Human?"

It might not come across perfectly, but she means it in the highest praise. Only a second later does M'gann realize staring is a little rude, as she tries to cover it with a little more conversation. "I-I mean, yeah. Yeah, I'm an alien; from Mars?"