Why do I know that?

By this point Grace is already suffering bad from malnutrition. Maybe juust at the "can still walk and talk but I'm sleeping damn near all the time" stage. Simon's sub is just straight up floating in space not far from this weird ass moon that visibly has way more color and stuff going on than its planet. They get close enough that Rocky can, with focus, hear a heartbeat. They debate the matter immensely (they can't afford to stay or bring whatever it is on board, there's not enough resources; Grace wants to anyway) and settle on spending one painstaking week figuring out how to secure it to the ship.

Initially the idea is to get whatever creature in there out, but there's a couple problems with that, namely being a) they have no way of protecting it from the radiation of space, they don't have the capabilities of Rocky's ship to do that for them. b) it is after intense scrutiny and observation from a distance determined to be filled with some kind of liquid, according to Rocky c) the sub is, in Grace's words when he looks at it from the petrova scope, "visibly pulsing" and he really doesn't want to fuck with that. They spend very valuable time figuring out how to secure the sub to the ship, weaving enough xenonite to be able to do so. The last spacewalk Grace ever does is securing the straps they made around the sub back onto Mary. It sets back his health greatly.

The insanity that is landing on Erid. On one hand, they have this weak, dying malnourished human who they very urgently need to start synthesizing the right nutrients for because he could actually die any minute now. On the other hand, they have this horrifying fucking Situation of a sub where the liquid inside is just Full of the same blood the other creature has, and the pulsing is coming from this seemingly other human who has connected and fused to the sub with something like branches or vines. Getting Simon out of the sub is objectively easier. Treating his arm and wounds is an insanely hard task, but it is still easier than treating Grace. Simon yells and fights when he wakes up, but he's not as strong as them, well quarantined behind a barrier, and smart enough to figure out that they're trying to help him even if he can't understand what they're saying. Simon being brought to see Grace (but not touch or get close to, never, Rocky is insane with the need to protect Grace, especially since Simon has proven he can be violent and Grace is so so delicate right now) and realizes with slight horror that the desperate chirping is them asking him how they can help Grace. But Simon has no idea because he doesn't know this guy at all or what's wrong with him other than being hungry. How do you explain you can't just "give him food" you don't have with a language barrier like that.

I don't know if anyone has tried to go without eating Any sugar that isn't from natural sources (like fruit!) for longer than 3 days, but you will start to bug out. Grace getting antsy and chippy with Rocky but he has no idea why 😭 swinging between feral and depressed all because he's out of Skittles. Gets rabid literally any time they put something remotely sweet in front of him when food testing on Erid. I'm not sure this is something any Eridian would pick up on, or that Grace would even pick up on -- craving sugar is so baked into humanity by this point that it's insanely difficult to recognize. (It alone is what got Napoleon actually exiled, because he decided to block sugar imports from the general French public. He didn't last long after that.)