Thanks, I'm glad you're enjoying it! :)
And you're not dumb at all - that's actually Chloe's special hallowe'en costume, and it's one of the "hidden" events in the game :) To trigger it you have to have done Chloe's "transformation" event (with the assassin), Jessica has to be love level 3 or higher and it has to be the weekend. Then go to Chloe's room in the morning and you'll get the special event and unlock the costume.
Hope that helps!