Everyone that I talk to in Maar Gan mentions something to the effect that monsters are getting through the Ghostfence, and if I'm interested in helping out I should talk to the priests in the shrine or the volunteers at the tradehouse.
I've talked to every person I could find and talked through every topic I could choose, but nobody asks me directly to help.
Is this going to be part of the main quest later, or is it part of any single faction's quests?

no such quest in the game.