Board Thread:Naruto Discussions/@comment-24593592-20140716142357/@comment-24593592-20140716174838

Elveonora wrote: Kaguya may not be human for all we know. Even though she was stated to have been the first human with chakra. But besides Black Zetsu, not even Hagoromo knows everything about her.

I think Hagoromo knows his mother isn't human based on that conservation with Naruto.