Rocky talks wrong. It takes Adrian a few weeks to realize it properly, because it’s just on the verge of noticeable, but once they do notice, they can’t stop thinking about it. And sure, when excited or distressed Rocky’s always had a tendency to forget to speak in full sentences, but this is not the same.
On bad days his questions aren’t even grammatically a question- just a statement spoken with a different inflection. He remembers himself when called out on it, which should be soothing but it just makes Adrian worry more. (“Adrian go sleep soon?” “Was that question, question?” “Yes was question. Adrian go sleep soon, question?”)
On that note, his sentences tend to be missing certain unnecessary words. “Was question” instead of “it was question.”
In his defense, Rocky has been in space for literal decades- has been completely alone for most of that time and living with a literal alien for another considerable portion of it, so Adrian tries to not make a big deal out of it, but it does keep happening.
Then Adrian gets roped into building the alien’s biodome. Alright, “roped into” isn’t the correct word, because Adrian was the one to suggest it, if only to spend more time with their mate who in turn spends a lot of time with said alien. It makes sense for Rocky to be with Grace- while Grace can understand Eridian very nearly fluently by the time he arrives on Erid, he is physically incapable of speaking it and needs Rocky to translate to other Eridians for him. Adrian stars to learn the alien’s language too, because Rocky can’t be there at all times and construction moves so much quicker with another Eridian being able to translate. (They need to build him a speaking device at some point!)
About six months in to learning Grace’s language, is when it happens. Adrian comes home, notices Rocky who is preparing to leave and asks “Where Rocky going?” With the exact same inflection as a usual question but without the actual Eridian grammar for making it a question. Their sentence is even missing an entire “is” because it was just quicker not to say it.
Funnily enough Rocky doesn’t even notice it, he’s on his way to a very important thrum about the possibility of disseminating Taumoeba to other affected stars so he just hums about the purpose of his hurry and leaves. Adrian on the other hand stops dead in their tracks, having to re-evaluate their entire life, because all of Rocky’s unnatural mannerisms suddenly have a very simple answer. Rocky did it because talking to Grace was easier that way. And it seems to be contagious. For some reason Adrian doesn’t seem to be able to find it in themselves to care.