It predates aliens as little green men. And I'm sure the word is a legally recognized phrase. It's quickly becoming, if not already a slur though. Here's googles etymological breakdown.
The word "alien" originates from the Latin word "aliễnus," meaning "belonging to another, foreign, strange". It's a derivative of "alius," meaning "other," ultimately tracing back to the Proto-Indo-European root "h,élyos. This root also gives rise to the word "else". The word "alien" entered Middle English from Old French, and its earliest usage as a noun and adjective in the 14th century referred to foreigners or strangers.
Ya but.... Money