The title of this article is conjectural. Though the topic is found within The Simpsons universe, a proper name is not available.
Number 51 is an alien who was a former Stonecutter. His rank is more than likely a reference to "Area 51".
He sings "The Stonecutters Song" along with the other Stonecutters. It is unknown whether he joins The Ancient Mystic Society of No Homers.
Treehouse of Horror X
He is seen in the audience while Kang and Kodos are on stage.