Thomas Shey
Legend
You mean IT! The Terror from Beyond Space? I'd say that the inspiration there is about 1/100th what we're seeing with The Joker given the massive aesthetic borrowing, it's not even comparable. With IT! the similarities were basically very broad plot outlines and nothing which makes Alien unique. Like it's entirely possible Dan O'Bannon or Ronald Shusett had seen IT! among dozens of other monster movies and parts of it were bouncing around in they wrote the story that they later re-wrote to be the screenplay for Alien, but the similarities are shallow as hell. You can see them discussed here for example:
I really don't agree with their argument; I think some of the practical similarities are far more important than they do, and its shown by the fact there's basically no movie with much similar structural elements anywhere between the two (or, really, before "IT") in the SF/horror genre. I'm not going to claim there was necessarily influence, but I'd find it really surprising if there wasn't.
But that's really neither here nor there; the point is that the similarities wouldn't have even been noticed by most viewers of Alien because the prior movie was not well known and had come out 20 years earlier so many of the viewers of Alien would not have seen it to make the comparison.
Prior art is only visible to someone who knows the prior art.