I think the relationship between Simon and Adrian should be like the relationship of Markiplier with Lixian, specially if is in the AU in which Simon gets to Erid first and is given to the care of Adrian.
Like I say in other of my posts, I think that if Simon gets to Erid first, then the language barrier between him and eridians would still be present since they have no computer (eridians never created computers and Simon only have the old technology of the SM-13, and I don't think that computer can do much) to try and record each other word for something as Grace and Rocky did, so their communication is based purely in signs, gestures and body language, maybe using drawings and 3D models.
Simon, who may be sad that he can't hear another human, but happy that at least he is with another living, and what he hope is, peaceful being, speaks to Adrian as if he could understand what they say. He says something, then leave a space in which Adrian does answer with their melody and notes, or just stays quiet and steps on the floor or vibrates, and Simon interprets that however he wants. Something like:
Simon, saying his opinions freely: That is my opinion, but I could be wrong, what do you think?
Adrian, who also is there listening carefuly: *Vibrates two times and makes a shriek*
Simon, nodding and smiling confident: You are right, I'm never wrong.
Adrian also pretends to understand and have their own version of the conversation. They also know that Simon is waiting for an answer, so they try to have a good timing.
They just love each other company.
I think that it would be funny if Grace is keep away from Simon while Grace is recovering. Adrian may have try to explain to Simon what it was happening, with models about how there is another one like him, and how Adrian's mate is here too, but if Simon understood that or not is out to you.
One day, without any warning, Simon is in his side of the biodome -He could have his own biodome, and the eridians just make it bigger to add Grace part of the biodome - doing something relaxing, like watering plants or something (for me this man is always watering plants I don't know), he hears steps from behind and doesn't concern to turn thinking is Adrian and just goes with his routine.
Simon, watching at the tomatos that have been started to grow: I think we will have tomatos in a couple of weeks from now, we could do a salad, or sauce, what do you think?
Grace, who thinks that Simon is talking to him, but is too curious for his own good and gets close to see better: Wow! You were able to grew earthly plants in an alien planet? That is amazing!
Simon, hearing a human voice and an actual answer after months, just jumps in panic, and tries to put some distance between him and the stangers by brandish the watering can, just to look at a man that looks like a living skeleton wearing a cardigan and realizes that the wind could take this man, he can't be that dangerous.
Adrian and Rocky have a discussion about who has the closest bond with their humans. Rocky says that is obviously him, since he spends years with Grace, both of them know each other language and they saved the stars. Adrian argues that is not the same, because Simon and they connect without having any mission to motivate them, nor language to express themselves, just their feelings, intentions and wish to connect.
Neither would admit that each other bond and human is quite amazing, and that they are a little jealous -Adrian would like to have been able to speak with Simon with actual words from the start, Rocky would want to know that his connection with Grace would have born even if they have never had the need of saving the stars- but they can be very competitive and proud with this things so they never say it aloud.