Did Arya became No One? The scene with Walder Frey was confusing. She can do that trick with the faces right? au was it a mask & I completely missed it?

She never became no one, she never passed the tests and when she left it was pretty clear: "I am Arya Stark of Winterfell and I'm going home" She can't exactly be no one after saying that.

I don't know about the mask she used to kill Walder though, but she did that when she killed Meryn Trant too, so I guess she doesn't need to be no one in order to do it.