The goal here, though, isn't to recreate the Aliens genre, but to add an Aliens veneer to the D&D genre. Your solution here is to play to these D&D genre conventions. You've made the Alien something that can be spottedd, fought, and killed per the usual D&D conventions. It's D&D in space, with an Alien flavor.
And there is nothing wrong with that at all. However, if someone asks how to do Aliens, that's a different question than how can I do D&D with Aliens. Hence, you'll get answers like , "D&D doesn't do Aliens well, try the Aliens RPG." This is an honest and valid answer to the former question. The problem is that a lot of people are actually asking the latter question, maybe because they live D&Disms, or don't want to learn a bew system, or don't understand that different games can work very differently from D&D (obviously not all do) and so can provide very different experiences. All are valid reasons, alongside many more, but this is really a matter of hidden assumptions in the questions running into people that aren't making those assumptions.