I think all you really need is to trust in your players. They're smart and savvy and they're likely to pick up on it. You don't have to have a flashing sign saying 'Non-Binary' to do it. Through pronoun usage and subtle descriptive language, your players will understand.

And if they don't, that's okay too. As others have suggested, maybe another NPC subtly corrects them. Or maybe the party develops a relationship with the NPC and what wasn't obvious before becomes an interesting characterization.

With how you've described your setting and the casual nature of gender and sexual identity, I think anything more would seem out of place. Just let this NPC live in the setting, like it's no big deal.

I've got a couple NB characters and for one of them I use the Changeling race, which is a little stereotypical perhaps, but really serves that characters identity. In fantasy, where gender, sexuality, appearance, etc. can be, quite literally, fluid, we can explore these human experiences through allegory. Slaying a beast can be symbolic of overcoming mental illness, a shapeshifter can be symbolic of NB identity. Or whatever.

Sorry, it's late and I'm very tired and rambly, but I hope this helps in some way. Basically, you don't need to overthink it. Just decide 'hey, this NPC is NB' and move on. It can be as obvious or subtle as you want. If your players don't immediately pick up on it, then, in a way, that is a success, as that identity has integrated and normalized itself in your setting. And if they do, it'll be a cool and affirming moment. Trust them to engage with your world and they will surprise you every time.