Aliens don't even have a sun on their home planet, it's all eternal night. And then we have people like you trying to shame them for wanting to get a bit of sun and beach activities.... smh heartless.
On a serious note, did this occur because of a bar night? Aliens are supposed to be SUPER rare unless you seek them out, just like mermaids.
The bar night however tends to make them spawn everywhere. I know this isn't ideal, but download MC command center and you have two choices. You can either disable alien bar night, or force them to hide in their disguises so at least it's not immersion breaking.
You might even be able to disable them completely through MC , not sure.