
It's not so much that he's jealous (a guy getting jealous that another guy is openly hitting on his girlfriend is justified, and kinda cute). It's more that she can't even be seen talking to another guy about normal things without him going over the top. The desire to isolate someone you love because you don't have the confidence that they won't leave you at the first chance is very dangerous. And if they are the type of person who's that fickle, probably not someone to get involved with.

Of course he’s scared. He’s scared of being abandoned or that she will leave him for others and forget about him. Regardless of what form he is in, he has always been possessive of Amy and clung to her, although he’s lightened up to Diana and this fact will always be imbedded into him, the fear of being abandoned. Y’all honestly have to be in his shoes. He may be a grown man, but his mentality is totally not okay. The way he thinks isn’t because of the environment he was brought up in. I get what you’re saying, but I see why Nox feels that way, it isn’t right, but it’s just a product of his emotions. And even if he gets his memories back, he will still want to stay with her, getting his memories back doesn’t erase the memories he had with her and the fact she gave him a name. And of course he’s not someone to be involved with MFOAOAOAOOOA HIS BACKSTORY GIVES US THAT INFO. The character development will come and Amy will teach him things like that are not okay, hopefully or he learns that she can have friends the hard way. No one calls it cute, they just see where he’s coming from, seeing how the situation is with him. He does need to dial it down A LOT.

okay, im going to fill the page with x's first in case anyone responds to this so the spoilers aren't revealed. [........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................]
grey is super excited that he finally met nox, and he can't wait to tell the others who serve under him that their leader is safe (albeit with amnesia). however, you see a bunch of shadowy figures talking about how obvious it is that he found his master (because his face is so expressive) and how he won't make it there to tell his peers that their master is alive. it's a bit more pertinent to the overall plot so that's why i'm making sure people who don't want to know too much won't see it.
Okay, it's not cute that nox wants to kill grey or that he's scared that amy is talking to other people. he literally has no memory and knows nothing other than her, it's sad.
grey is a good guy. he genuinely cares for nox (it's not a facade) and wants the best for both nox and his fellow colleagues who work under nox. nox freaking out and wanting to be the only one amy talks to is not cute. that is toxic. luckily they talk a bit more soon but let's not romanticize possessiveness from a grown ass man in a bad mental state (he once again has literally no memories, that can really mess you up).
spoilers (none plot shattering)
he gets mad when she calls grey by his name, and gets angry thinking about her taking care of other people. she basically tells him her sister picked up grey and that he's from his previous life and wants nox to talk to him so he can learn more about himself. nox and amy communicate and kind of get on the same wavelength. he basically tells her he doesn't want to leave even if he gets his memories back - which is stupid, because like,, you literally don't know what person you will become when you get your memories back. amy knows this, of course, but tells him she'd never force him to leave and just wants him to get his memories back. there is some other stuff that happens at the end i won't get into due to it being pertinent to the overarching plot surrounding nox, but you can ask me if you want spoilers.