With that criteria in place I think @screenersam's answer of The Man Who Fell to Earth is the best answer so far along with @Tiran's The Day the Earth Stood Still. I don't recall if the Will Smith version of the movie explains why he looks human but if memory serves correctly the original never explains why an apparent human is the representative sent to Earth.Didn't Psychlos have to breathe "Breathe Gas" which was toxic to humans?
Some other ones to consider would be The Brother From Another Planet from the 80's and the Peter Cushing's Doctor Who movies from the 60s.
Thinking out loud... maybe the 1980 Flash Gordon movie? "B" movies eligible? If so there is stuff like Teenagers from Outer Space and Santa Claus Conquers the Martians (or was it "vs The Martians"... wait, was that a different movie? Hhhm....). Then there's B movies from the 80s like I Come in Peace with Dolph Lundgren.
In This Island Earth, were the white-haired aliens revealed to be anything other than humans with large foreheads? Yikes, I can't remember the details of that one, I may have to watch it again.