If you pay attention to the whereabouts of this dungeon on the map, you can spot a small lake. Go toward it and you’ll see some wooden platforms. They lead toward the dungeon’s entrance. If you are approaching this area from south, you will be at a significantly higher area above the dungeon. Jumping down to death, was worth for me, since going around might take too long.
We are now inside the dungeon, following a path to our right, toward the southwestern corner. There, on the ground, next to some shiny, probably silver, rocks is the shard. If you want to kill the dungeon boss as well, you’ll need to head toward the opposite side – to the northeast.
Sand Kingdom Moons - Super Mario Odyssey Power Moons