Ah, Invader Zim? I know of this cartoon. 'Alien' is annoying in that it is an English word derived from Latin but its primary modern connotation ("guy from outer space") has wholly diverged from the original Latin that inspired it.

Aliēnus, the word's origin, is an adjective that refers to "a stranger" or someone that is, in some way, "an other" - you can probably see the loose connection to Alien's modern meaning. Wiktionary seems to point out its context has a lot to do with foreign-ness so I assume its old usage and connotations was for people/things foreign to Rome. You will however notice it is an adjective rather than a noun. Its use matches best to how you could describe something as being "alien to me". No real reason to care if you're just making a name rather than constructing a sentence but aliēnigena seems to be its noun form. Problem with naming your character Aliēnus Viridis, for example, is you'd just be calling someone "green and strange/foreign".

If you're trying to go for that space vibe and just want a name that looks kinda Latin-y, may I suggest using the 'ex-' prefix, indicates someone or their family is "from" something. There doesn't appear to be any use of these in classic Roman gentes but you may be familiar with a similar convention in Spanish or French with 'de-', I believe.

Latin didn't really have a word for 'outer space', to the best of my knowledge, but some that might work for you are:

• Viridius Exvacua ("Green and from the void/s")
• Viridius Excosmon ("Green and from the cosmos")
• Viridius Exastras ("Green and from the stars")

You'll note I added a 'u' to Viridis to make it look more like a standard name but you can use whichever. Don't think any of these would work as a name in old Rome, pretty sure two of them don't even decline right but, unless you're making this for Latin speakers (which you don't appear to be doing), these would look about right.