Marie didn't have a costume on. What the hell was she going to wear anyway? She wasn't good with pop culture and she knew mostly old novels and while she could have dressed from that, the scariest characters she could come up with here normal looking. That was what made them scary. So maybe she was one of them.
Or maybe she just didn't have a costume. She did have a cup though, holding it close to her chest as if it was something to do. She had spotted him, but didn't approach. Maybe he wouldn't notice she was here and she could just sneak out after a little while.