Nox would stay behind in the city to assist with search and rescue efforts. He does have the benefit of the Force after all, so he'd be able to sense anyone alive trapped underneath any debris or rubble, as well as have the means to help with any kind of heavy lifting to get to them. Once that's over and done with, he'd move on to helping with any kind of rebuilding / repair efforts.

Naturally, he'd also try to find out more about the culture and history of Atlantis during his stay, if that's something the selkies would humor him with. He'd try to learn more about them as well, but since xenobiology isn't remotely in his wheelhouse it'd be more in the way of general knowledge than anything too detailed (e.g. how long they usually live for, what's their diet like, etc.)