I was in a horror movie Facebook group awhile back and some people there would get angry if you misclassfied a film as a slasher if their "rules" said it was actually "splatter" or "psychological horror" or whatdafuk. I didn't take the time to try to understand it because I didn't care. So, I just left the group.
What I'm trying to say is that I totally think that "Alien" counts as a slasher film.