Suppose there was a species that was very peaceful, very good at diplomacy and just generally very nice— but they also happened to look really terrifying to humans. Sort of an opposite to that ‘humans are cute space orcs’ thing— species X is perfectly friendly, but just happens to look like they walked out of a human horror movie.
We don’t blame them for it, it’s not their fault (and we’re slightly too afraid to talk to them about it anyway) we just quietly avoid ships where they are stationed and stay away from areas where they live and, over time, it just becomes accepted that, for whatever reason, you don’t put humans and species X together. Captains turn down human applicants if they’ve got a member of species X on their crew and visa versa. They barely notice that they’re doing it, it’s just how things are done.
Then one day a human crewed ship breaks down in species X space so that one of their ships picks up the distress signal. Being such lovely people, they offer to help and the humans can’t think of a good enough excuse to refuse.
The repairs take about a week and, the whole time, the species X crew members are loving the human ship. It’s so spacious, you barely even see other crew members! (They don’t realise that all the humans are constantly ducking out the way whenever they see them coming.)
The humans, meanwhile, just spend the entire week in Hell. The species X crew members like to take shortcuts through the ventilation shafts, so you can constantly hear them skittering around above your head; the ship is full of this low key but very distinctive smell— rotting meat, the smell of death (apparently they give it off when they’re happy); half the crew have goosebumps, despite the temperature controls working perfectly.
The ones working in the engine room directly alongside the species X crew have it hardest though, they can’t run away— and it’s very hard to relax and do your job when, suddenly, you hear this noise above your head and a hairless, milk white creature with no eyes and a huge mouth filled with razor sharp teeth and long gangling limbs with fingers and toes that look human but like they’ve been stretched, leaps silently with catlike grace from the rafters, lands right next to you, flicks out a forked tongue, holds out a long taloned hand and asks “can I borrow your spanner?”