3-Up Moon Block
From the Super Mario Wiki
The 3-Up Moon Block is a block type that is introduced in New Super Mario Bros. U and reappears 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 in Boost Mode. The player must tap on the block to make it appear, and players using a Wii Remote need to hit the block to obtain a 3-Up Moon quickly, as the block will disappear soon. One 3-Up Moon Block can be found in every level.