Roots Search: Shokushin Buttai X

Both are criminally underrated 80s sci-fi horror anime. In both mysterious things happen onboard a space craft with a lot of murders as a hidden threat takes out the crew one by one. Very similar tone and pacing, though the developments are very different. Both anime feature a paranoid and frightened crew as they deal with an unknown danger, really good stuff, and if you enjoyed one you'll likely enjoy the other. 

Both are old and fairly obscure sci-fi horror where the alien is some sort of god-like entity that uses psychic powers to torment and kill its victims one by one, making for some entertainingly strange scenes,as well as having an odd but interesting musical score. 

Both are criminally underrated pre-2000 sci-fi horror anime. In both mysterious things happen onboard a space craft with a lot of murders as a hidden threat takes out the crew one by one. Injuu Alien is a pinku (story-based hentai), lots of tentacle rape. Roots Search is a relatively less explicit (but still absolutely not for children). Both anime have very similar plots and stories, and the same sense of old-school sci-fi horror. They're both from different generations, Injuu Alien from the 90s and Roots Search from the 80s, but they close the generational gap in how similar they feel.