Alien, as in extraterrestrial, as would be described by a Mexican laborer living in South Texas. Both plural and singular. Thank you in advance for your expertise.
La Extraterrestre
Plural: Las Extraterrestre
or, if you prefer
La Alienígena
Which one might depend on who he is talking to. I'd go with the first.
This, except you got the gender wrong, it's "el extraterrestre" and "el alienígena".
"ajeno" is for "a stranger/an object that belongs to someone else" and so on.
Extraterrestre is more likely to be used by a farmer type as alienígena is more formal/scientific.
ETA: plurals are "Los extraterrestres" and "Los alienígenas"