Morellium!!

Grace who does jazz hands and taps when he's asking a question and says some words three times like Rocky. Grace who makes chirps and trilling noises when he's happy.

Grace who starts thinking and dreaming in Eridian, who sometimes has dreams that are just sound, no visual. (Grace who, if he ever goes back to Earth for whatever reason after years on Erid, forgets some words in English and speaks in the sentence structure of Eridian because he has to translate from Eridian to English when he speaks outloud).

Grace who does scarification (even if Rocky protests at first) - I've read some fics where he does the Voyager pulsar map, or dots and lines showing his route from Sol, to Tau Ceti, turning around towards Sol, and then the line going all the way back to Erid. Even though he already has the scars from Rocky saving him, maybe Rocky's family crest. A scar showing his rank as Captain of the Hail Mary - maybe something to represent her, too.

Grace who wears Eridian gems, as earrings, in a chain for his glasses, in strings draped over him that clink together when he walks - the most important ones, like the earrings he wears all the time, the color of the blueish-green gems (or spots?) in/on Rocky's carapace.

Grace who wears a bracelet on each arm that have ribbed marks like on Rocky's arm so he can properly say goodbye (not something he had to do, but something that felt right to do, something that made him feel more like himself), sound included.