Entrance to Hightide Hollow is to the east of the marked icon on the map. There is also Hjagir NPC quest mob that starts the quest “The Truth about Spiders”. As you explore the dungeon, you’ll come across the first bridge. The shard is just beside it, next to the lava and the dungeon walls. It is hard to miss, as the main (and only) mine path takes you to this point.
Hightide Hollow Entrance.
Hightide Hollow Entrance Map Location.
Couple of beasts nearby.
Sand Kingdom Moons - Super Mario Odyssey Power Moons