All throughout "Don Juan", the Phantom is wearing a mask that goes just above his eyes, and ends at about the bottom of his nose. We can see that he has a clear forehead and the bottom of his face is entirely clear as well. His hair is all there, too. However, when Christine takes his mask off, the entire side of his face is deformed - including the forehead that was so clear just moments before! And his hair is also receding, which would have been visible even with the mask on.