Creating the Full Moon Island
When you reach the island, it only looks like a crescent moon. Go to the pier in the north and throw out the fishing rod. Pull the sidesteppers out of the water and defeat them in the subsequent battle.
After the victory, an earth-folding circle appears. Use it to lift the missing part of the island out of the sea. This creates the full Full Moon Island.
In the treasure chest you will not find the searched key, but only a mushroom. You also discover a message from Luigi who has already taken the diamond key.
Note on the diamond key
The note reveals that Luigi is waiting for you on Mushroom Island in the vast ocean. There you can get the diamond key that is needed for the diamond island.
If you wish, you can explore more optional islands beforehand and discover additional treasures.
Follow the solution either to Mushroom Island or you will first visit Scuffle Island.Mushroom Island
Treasures
? Blocks
Holes