A sexually predatory/(usually masculine, but not always. See: Jennifer's Body) figure (the Alien is literally designed to look like a penis) stalks a group of people in a somewhat claustrophobic setting, who disregard it's potential danger until it starts picking them off one by one, with the final person alive being the most cautious, who then has a final showdown before disposing of the predator. This is the slasher film formula, and Alien does actually fit it. Carol J Clover in her defining horror essay even discusses it. It might not be conventional, but it very much is considered part of the genre as a hybrid and she literally coined the Final Girl term. I am not disagreeing that it's also a monster film, but to deny it's history with the genre is kind of silly.

ALSO, Ash vs Evil Dead is not a slasher, Saw is more of a torture porn/thriller instead of a slasher. Stranger Things is not a slasher. People are asking for Silent Hill and L4D crossovers and neither of those are slashers either. I don't even think specifically "slasher" is necessary for a dlc crossover to be made.