The Great Immortal fears humans backs? Is it because they are hairless or extra hairy? After all immortals always got super long hair covering their back to their butt. Backs are probably secret.
it will be explained in later, the earth he knows before is not the same earth he is discovering right now. he's trying to stay lowkey.
to gauge the true power level of the earth. its all poppycock though. i mean...worried over nothing. that sort of thing. he doesn't wanna spook any unnecessary powers that might know the real existence of a real immortal etc.