To find an entrance of this dungeon, you have to reach the lower level in this area. If you find yourself above, just follow the waterfall down the stream. There are small ledges you can jump from, and not get hurt too much. The front door of this dungeon almost perfectly blends into the mountain. I would totally miss it, if there wasn’t the map marker. Just approach it, and the interaction option will appear. It is close to the waterfall, next to some lit torches.
This is a rather small dungeon. The shard is in its dead end room, southeastern corner, in a small pond. If you are afraid of the spiders, you won’t like this dungeon, as it is full of them. I guess I was lucky, since someone had already killed them before I’ve reached the area.
Sand Kingdom Moons - Super Mario Odyssey Power Moons