Few questions about Moogles in Churning Mists

[Cilia]

Were they "born" there? Or they're just survivors from the Fire era?

Without knowing the average moogle's lifespan... tough to guess. Going on some later quests up there I'm pretty sure they're not ageless, though, so I would wager that they were in fact born up there and are descendants of survivors from the Calamity of Fire.