3-Up Moon Block
From the Super Mario Wiki
The 3-Up Moon Block is a new block type that is introduced in New Super Mario Bros. U. This block is very rare, hidden in places that are hard to reach, and can only be found by using the Wii U GamePad. The player must tap on the block to make it appear, and the player(s) using the Wii Remote need to hit the block to obtain the 3-Up Moon.