This collectible is close to the main road. You can spot it easily from a nearby wayshrine. There are a couple of large boulders that might obstruct your view of it if you arrive from the east, so be careful not to miss it when coming from that direction.
From Fort Grimwatch Wayshrine. turn east. You can spot the collectible in the distance.
Close to main road, next to a couple of boulders.
Collectible map location.
Sand Kingdom Moons - Super Mario Odyssey Power Moons