"a white wizard" should be accepted because "the white wizard" implies that he is some specific white wizard (either the only one in existence, or Gandalf the White mentioned earlier), and the sentence only means, most likely, that he is white rather than gray.
I didn't try this but i think it should also allow "or is it the white wizard?" And "or is that the white wizard?"
(I.e. is it the white wizard that walks in the forest).
Both would be a more natural way to say this English than "he"...