Alright, new headcanon where eridians get to flip amatonormativity on its head
Friends live together, and romantic partners, if any, are someone you go out to hang out with.
Friends are more plentiful, naturally form groups well based on common interests and experiences (particularly if they grew up together, such as spending years in school together). Romantic relationships are... maybe not chemical, but whatever the eridian equivalent is for sudden, intense infatuation, that lacks rationality and long-term stability. Other people you get along with aren't likely to have the same chemical response, so it doesn't form groups well.
Eridians have 5 children per clutch, which is a lot, takes a village to raise — not to mention needing someone to watch you sleep (and more people are less likely to have sleep overlap), and someone to watch pebbles.
So, when Rocky calls Adrian his mate, he means it in the combination of "friends" and "life partners" and "coparents" — because he learned bits of those meanings from different sources, but they all made sense together in his culture. He didn't realise British "mate" is considered completely different from animal "mate".
When he left, he only had one mate, but they expected to join more. He worries that if he's considered dead, Adrian might have joined a mate group that didn't plan space for his inclusion.
(possibility the word he uses for "mate" is also an uncountable noun (like "water" or "love"), and can refer to a mate group — but in the sense of the feelings, care, and support, rather than the individuals. It makes it sound like he's worried Adrian found a new [singular partner], but he means a whole found family)
I like the idea Adrian did form a found family with the mates left behind by other crew members
But I'm also thinking about how they'd perceive Rocky's relationship with Grace.
The intensity and irrationality of it is something they'd associate with romantic relationships, and are worried about it destabilising the mate-group that's not likewise interested in Grace as an emotional life companion, if he were to be integrated.
Others might see it like Rocky brought home a pebble for them to care for together, and welcome him in the limited capacity they can.
It works out that caring for Grace means providing a space with his atmosphere and enough room to roam. Rocky can visit him outside the mate-group's home, Adrian can care for him on the biodome team, and Grace gets visits and socialisation from the other carer-inclined members that don't have biodome skillsets.
