Naming Conventions of Keeper of Moon Surname?

[Iscah]

I was there but there is no clear guideline on what a name would be. It is also stated there are names that have not been seen yet. So it feels as if you could just put whichever surname you wanted and it'd be accurate unless I'm thinking incorrectly?

That's where the random name generator (at the last step of character creation, after selecting a world) comes into it. It produces the correct naming format for the character you're creating, so everything it will suggest for a Moon Keeper is a lore-accurate Moon Keeper name. So just keep generating names to see what surnames come up, and pick one that you like from that list. You're free to overwrite the name after it's been generated.