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 and reappeared in New Super Luigi 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 players using the Wii Remote need to hit the block to obtain the 3-Up Moon.