When I hear the words “space alien,” I admit I immediately think of little green men. Stereotypical, I admit. I’ve read many teen books where you know from the get-go it’s an alien because the alien doesn’t look human. He/she/it is often an exchange student or visitor from another planet, and the action takes place on Earth or on other planets. Kate Gilmore’s Exchange Student, Allison Goodman’s Singing the Dogstar Blues or Annette Curtis Klause’s Alien Secrets are examples.
I prefer books where the aliens pass for human and the characters don’t know that there’s an alien among them.
To quote from I Am Number Four by Pittacus Lore:
We look like you.
We talk like you.
We live among you — but
We are not you.
We have powers you dream of having.