What's the difference between many religious concepts of gods and an extraterrestrial civilization that (at least on our scale) appears immortal, omniscient and all powerful?
Two major differences:
1. Evidence. Extraterrestrial life is definable and testable. "God" by most definitions is unknowable, and most religious definitions of "god" and his/her/their/its work are contradictory to scientific evidence.
2. Purpose. Nobody has committed genocide in the name of extraterrestrial aliens, but religion has been used to justify all manner of atrocities.
Goodness, that's some fairly shoddy logic for point two! I could equally phrase this as "No religious group have committed genocide in the name of extraterrestrial aliens, but atheists have been known to justify genocide".
In other words, the first assertion includes everyone, including ALL religious people, and the second premise doesn't follow from the first anyway. Furthermore, the argument doesn't even answer the question between the differences between aliens and deity, because you are focussing on the religious movements and not the omniscient god!
Two major differences:
1. Evidence. Extraterrestrial life is definable and testable. "God" by most definitions is unknowable, and most religious definitions of "god" and his/her/their/its work are contradictory to scientific evidence.
2. Purpose. Nobody has committed genocide in the name of extraterrestrial aliens, but religion has been used to justify all manner of atrocities.