Khartag Point entrance is approachable from the seashore side. It is hard to miss, as there is a small pier outside, and other small elements that point toward the human factor in the area. Find Nakhul nearby, he will give you the dungeon quest “A Smuggler’s Last Stand”.
As you enter the dungeon, keep to your right. Keep going this way, until you spot the second turn on your left. There is a long cave room, just before you reach the flooded area. There is a bright blue color coming from the crystals at its entrance. At the end of the tunnel, you’ll spot the red crystals, and their glow.
Sand Kingdom Moons - Super Mario Odyssey Power Moons