The Doctor is horrified at the site. Partially because, yes, their bodies are falling away. But mostly because he never expected to see something like this in dreams. It's horrible. While he has his own nightmares, he wonders if this realy is a nightmare for them. Maybe it's what they want. Which makes it all the more terrifying.

He raises a hand and simply gulps down the feelings in his throat. "Are you alright? Of course you're not, you're rotting! How can you be alright? There has to be a reason--why are you--what happened?!"