Wait, pixie can changes? I dont know about that did i miss some quest or clue?
Lol nevermind, I remembered. I decorated the bedroom for Halloween a second time.
That's Pixie's Hallowe'en costume :) Make sure you have all the Hallowe'en decorations active in your room (Jack o' Lantern, Coffin Bed and Witch Poster) and then sleep. It might take a couple of nights.