I wanted to come back and share my headcannons about Iron Lung and Simon a bit so you guys could play with that if you want too so here we go!!
To me, it seemed like the blood ocean was what remained of humans after the quiet rapture. As if maybe some went somewhere else and a lot of them returned to this primordial ooze state of collective consciousness, with all of their wants and desires all mixed together. Watching the movie had me thinking that Simon would have been fine if the hull hadn't breached and the blood hadn't gotten on him. Because it did though, that's what gave him more of a connection to the eel and why he started seeing everything he did.
The eel is canonically fused with the bodies and minds of the crew Simon thinks he's speaking to so I think it started as a collection of the first humans trying to come back to form. They formed an eel which seeks out physical forms to become "whole" with or "right/more human," with.
With this headcannon of eel!Simon I figured somehow in the creation of the blood tree, his own consciousness was thrown to another part of the universe or another one entirely by the same cosmic "light" that he saw in the ocean. It's like it was his own private rapture, a, "congratulations! You survived the trial of the blood ocean and spawned a new tree of life for humans/life to eventually grow from once they're ready to live again! Here's the consolation prize for going through all of that! A new, hopefully kinder universe since this one is done with you! Ok, Bye!"
It seems like something a nameless, formless, cosmic entity would gift someone with. The light wanted to be seen and with the black box recovery at the end of Iron Lung it will be. So, even though Simon was only a servant of the Light for a short time, he fulfilled his purpose and was given a gift.
While the voices/feelings from everyone fused with the eel are gone from his own consciousness, Simon was still a part of all of them/that to be forever changed by it. Maybe another gift of whatever God the light was, or just a byproduct of being a part of the blood ocean/the new world tree (blood tree) as long as he was.
Now that he's on his own, consciousness freed, that blood within him is amplifying his emotions. He doesnt realize at all at first because he's been through a lot in his own universe. Of course he's mad, of course his emotions are elevated but wait, was he always this strong? Could he always see this well in darkness? How can he seem to sense Grace from so far away or even while he himself is asleep and dreaming? Why does he find himself starting to circle grace like prey more often than not when they're in a room together? Or lurking in the darkness of the hall and just waiting and watching Grace as he works for who knows how long?
I think it would be so fun to see Simon struggling to hold himself back/keep himself in line like he was in Iron Lung. The more freedom he gets in this universe and around Grace, the more he feels relaxed but also, the more he feels his control slipping. So he's constantly at odds with himself because his body and subconscious are like, "we're free! We deserve this! Let's connect to another person in any way we can!" And the self hating, I don't deserve kindness side of him is like, "please don't hurt Grace, he doesn't want me, I don't want to hurt anyone again."
Meanwhile, Grace is trying to just not let Simon die and trying to figure out how to help him because yeah, he's human shaped most of the time but he is absolutely not human according to any blood tests, or anything he's seen looking back on the ships security cams or anything.
Then of course, of all if this is happening on Erid post Project Hail Mary, Grace will have the added bonus of telling the Eridians, "no, humans shouldn't transform like that physically. No, they shouldn't be able to communicate telepathically. No, getting cornered and softly bitten at by someone's newly formed, razor sharp teeth is not a mating ritual that any human has ever been a part of."
But still, again in my headcannon, Grace is pretty touch starved. A graze of fingers against him is enough to send a shudder through him and now he has a velvet voiced, absolutely beautiful, deliciously muscular, charmingly rougish, gentle monster/man pinning him against walls or tables and whispering all manner of filthy things into his ear during any of his random transformations, no matter how partial they may be at times.
