I've been doing my own investigation into the matter and this is pretty legit.
You can see whenever Nightmare appears that his whole body isn't there if you know what you're looking for; even on the bed it's obvious that his head is working differently than Fredbear's.
You can even brighten up the image whenever he shows up to reveal that he's mostly transparent.

What are you trying to tell us Scott…?

I think that's his endoskeleton head. Not 100% sure doe.