An extra terrestrial alien head, using the stereotypical depiction of an alien with a bald head, large open eyes, and a mouth.
iOS displays an alien in a box as a placeholder for an unknown emoji character. That symbol is unrelated to this emoji.
Alien was approved as part of Unicode 6.0 in 2010 under the name “Extraterrestrial Alien” and added to Emoji 1.0 in 2015.
👽 Extraterrestrial Alien