You're not supposed to refer to illegal aliens by that term, even though they are foreign nationals (aliens) who are here illegally.
Some prefer the term "illegal immigrants", but in recent years even that has come under fire. In theory, the politically-correct term has evolved into "undocumented immigrants" - but that's incorrect as well, because immigrants refer to their new country as "home", while most illegal aliens send money home.
Thus, the latest terminology is "undocumented workers". Please make a note of it.
But there's no need to stop there - as one wag noted, in today's news, a gun battle occurred between a homeowner and an unlicensed invasive extrajudicial wealth redistributor.
But this raises another question: how do we properly refer to the folks who are busily taking your money to build trains and trams that nobody wants?