HEY SO what if. hypothetically. eridians constantly emit high levels of infrasound via their thrum organ, and what if it makes grace sick, and what if it keeps getting worse the longer he's on erid :))
if you dont know: infrasound is low-frequency noise generally not audible to humans (below 20Hz) but can be felt as a humming vibration at higher pressures. grace is comforted by it because it reminds him of rocky! and rocky being there! only it also penetrates biological tissue, can potentially damage the body on the cellular level and cause severe physiological & psychological effects, especially when exposure is chronic/long-term. source (the field is very heterogenous & i'm no expert so take all this with a grain of salt)
“Symptoms attributed to the effect of infrasound include, but are not limited to, headache, concentration deficit, mood change, depression, sleep disorders, pulsation and panic disorders [...]” source
it might even impact cardiac function and lead to fibrosis (tissue scarring), and you don't need me to tell you that heart disease and organ damage are bad.
anyway. what if grace recovers from the starvation only to deteriorate again because he is constantly near eridians and even when he isn't, infrasound travels far. he can't sleep, he has brain fog and mood swings and balance issues, his blood pressure is elevated, he always feels on edge. he knows something is Deeply Wrong, and he gets worse and worse but neither him nor the eridian scientists can pin it down because it's just not on their radar!
and when they eventually do figure it out, they make the biodome fully soundproof, of course. but some of the damage is irreversible, and also: he cant interact with them as much anymore. he has to limit his exposure. he loves rocky and his people but they're essentially slowly killing him just by existing.