As the word "Alien" describes what all foreigners are I can see no objection, if I cared, to it's use! It is a just a word as explained here;
From the Oxford English Dictionary;
alien
n adjective
1 belonging to a foreign country.naturalized.
2 unfamiliar and distasteful.
3 relating to or denoting beings from other worlds.
n noun
1 a foreigner, especially one who is not a naturalized citizen of the country where they are living. Øan alien plant or animal species.
2 a being from another world.
ORIGIN
Middle English: via Old French from Latin alienus 'belonging to another', from alius 'other'.