If you look closely on your map, you’ll spot a beige colored structure at this area. Head toward it. If you are aware of the shard in this location, and you don’t bother with reading the guidelines, you might end up entering this building. There is a rather long underground set of tunnels called Exile’s Barrow beyond those doors. You don’t want to end up exploring it, if you just want the 1/3 of a skill.
Instead of going into the ancient barrow, find an uphill path just east of it. Ascend on top of the nearby hill and look down on the building once more. You should now spot the bright beacon you are after. It is just behind a larger stone boulder, close to some annoying harpies.
Sand Kingdom Moons - Super Mario Odyssey Power Moons