While Harley was busy giving her little speech to Elle, Crowley was moving closer to her. Maneuvering himself so that he was behind her.
Why was she wearing such a stupid and trite costume? Did she choose it because of her name or did she choose her name based on her costume choice? It didn't really matter, he decided, because he wasn't particularly fond of the game she was playing.
The distraction game. It was ruining he and Elle's siege on the castle. Or at the very least delaying progress. Neither of which was Crowley pleased with. So, he had the notion to distract Harley so Elle could continue on her mission.
Which should be easy enough once he ripped the mask off her face. That ought to get her attention real fast and put it on him. Wasting no time, he made the quick attempt to unmask her. For some reason, she felt the need to hide behind a costume and Crowley didn't fancy her having that advantage.