At one point in the film if i remember rightly he mentiones that he appears to be human beacuse he is on a planet inhabited by humans and that he would appear differently to different species that inhabit different planets such as his own, but if this is the case then why would he need a human body to appear human on earth?
this could either be a plot hole or a intencional slip-up to back up the evidence of him being human!
I don't think it's a slip up at all
It's clear that the physical body is of Robert Porter (Think that was the name), a guy who was so disturbed/shocked at finding his raped daughter/wife, he went into a coma type state (I forget the name)
So "Prot" uses his body as a host, to experience and travel around earth, which is why he said, I look like a human on earth, because it's his spirit/mind/presence which is inside a human host, likewise, when he leaves and goes back home, he reinhabits his alien body on K-pax.