Moon Rock

From SMO.wiki

Moon Rocks are grey/silver cubes found in most kingdoms. The only kingdoms that do not have moon rocks are Mushroom Kingdom, Dark Side, and Darker Side. When hit with Cappy or ground pounded after the game is completed, moon rocks will release a lot of extra power moons into the kingdom they are in. These power moons will be immediately marked on the map, making them somewhat easier to find than other power moons.