It got really dark around him and there were evil whispers of the kind that Peter Jackson put into his overlong Hobbit trilogy when Gandalf was at Dol Guldur.
Sauron wants people to think he's a fallen star, which I suppose is some form of Maia in Tolkien's world, and if he easily fools the Harfoots and uses them to his benefit, he will have contempt of halflings that will carry over into the Third Age and make him underestimate Frodo.
I suspect the whole "sort of looks like Gandalf" thing is to fool viewers who aren't very well versed in the appendices to the Return of the King, the Unfinished Tales, the Silmarillion and the 12-volume History of Middle Earth that Christopher Tolkien published.
Or they fucked with canon in a major way and you're right and he is Gandalf, just a bit darker and more prone to destructive magic than later on.
ἡ δὲ κἀκ τριῶν τρυπημάτων ἐργαζομένη ἐνεκάλει τῇ φύσει, δυσφορουμένη, ὅτι δὴ μὴ καὶ τοὺς τιτθοὺς αὐτῇ εὐρύτερον ἢ νῦν εἰσι τρυπώη, ὅπως καὶ ἄλλην ἐνταῦθα μίξιν ἐπιτεχνᾶσθαι δυνατὴ εἴη. – Procopius
Ummaka qinnassa nīk!
*MySmiley*
