Board Thread:Wiki Discussions/@comment-1512913-20140521221412/@comment-28861328-20140524213611

It is a Sharingan. The panel right before Hagoromo says that, talking about the Infinite Tsukuyomi, is showing nothing but her third forehead eye. It is delusional and a complete rejection of reality to suggest that Hagoromo was talking about some mythical mysterious, un-seeable eye, or that he was referring to the Rinnegan by blatantly calling it a Sharingan. Furthermore, Obito explicitly says that one must cast their Sharingan (when he was explaining the plan to the Five Kage) on the surface of the moon, after becoming the Ten-Tails jinchuriki, to cast the genjutsu.

That makes the third eye *drumroll* a Sharingan, backed up by Hagoromo's own statements about his mother.