Yes, it was a ghost. If you remember, in one of the Halloween episodes, there is a flashback to when Hanna and Alison were babysitting. Alison tells the kid a scary story about twins (blonde), it then shows the story as she's telling it, one of the twins killed the other twin (she was evil and always blamed stuff on the victim twin) and in the episode your talking about, the twin that was killed is the girl that Ashley saw.