Allan Hynek and Jacque Vallee eventually came to the conclusion that these so-called extraterrestrials were actually extra-dimensional.
I listened to J. Allen Hynek speak (he toured as a sort of follow-on to the movie CEOTTK), and he suggested “metaterrestrial” as one possibility — that is, Earthlings but from the future. The problem with that, or rather, one problem with that is, the Earth is moving, and thousands of years from now won’t be in the same place. :’)