Seeing this question pop up today led me to wonder if the aliens should be migrate to something along the lines of extraterrestrials. This is mostly because I tend to associate seeing with "alien" with the "Belonging to a foreign country or nation." definition.
English is not my first language, so my perception may be different from people for whom English is a first language. But I think that it's mainly an American phenomenon, when I spoke with British people about the subject, they didn't use the term aliens, but illegal/undocumented immigrants. For me alien is a more instinctive name for an extraterrestrial than extraterrestrial. When I watch American movies/TV I sometimes need to remind myself that they are talking about Mexicans and not Martians, as I'm genuinely confused.
I'm afraid that some people who are not familiar with the term extraterrestrial, might miss it.
However, there is a different issue, which is the tag ufo, all the questions under this tag talk about extraterrestrials, but they are not all tagged as aliens as well, which means that there are questions that are missing their appropriate tag, I suggest instead to merge the ufo and aliens tags under the aliens tag.