To find the shard, you need to reach the western part of the dungeon. You have to go downstairs, as this part of the cave is on a lower level. There is a rather large rectangle-shaped room in this area. What you are looking for is in the southern part of this room.
Find and enter Viridian Watch solo dungeon.
Viridian Watch map location.
There are no enemies nearby.
Exact location on the dungeon map.
Sand Kingdom Moons - Super Mario Odyssey Power Moons