If you come to the dungeon’s area, you’ll spot a vast number of stone arches. Follow the path underneath them, until you reach the door that leads into the delve.
The Garlas Agea shard is found in its southeastern part. It is not hidden by any means. You’ll spot it as soon as you enter this room. It is located just next to the food cart, close to some of the dungeon inhabitants.
Sand Kingdom Moons - Super Mario Odyssey Power Moons