So if Mel met the real Arya, how would she mistake Alys Karstark for her later?
Neither of their names are spoken so Mel doesn’t know that she is Arya Stark. Not only that but I don’t think Mel ever actually sees the face of the girl riding to the wall in her fires. If I remember correctly she only sees a grey girl( or a girl in grey?) on a dying horse.
For half a heartbeat she let herself pretend that it was her home ahead. But that was stupid. Her home was gone. Arya never seemed to find the places she set out to reach. ~Arya, A Feast for Crows~

(Fanart by しう)
Things like this are why I really, really want to see a Stark family reunion at some point in the books. (Yes, I’ve read all of them, so I’m well aware of *SPOILERS*.)