Blockhopper
From the Super Mario Wiki
Blockhoppers (also know as Coin Impostor) are robotic enemies encountered in the game New Super Mario Bros., found in World 2 (the desert world). They disguise themselves as harmless brick or ? Blocks, but once Mario approaches, they reveal their true form - blue robot-like enemies - and hop at Mario in tune with the background music. They can be told apart from ? Blocks by the fact that the question marks do not move. They are similar to the Pile Driver Micro-Goombas that disguised themselves as blocks from Super Mario Bros. 3. They can be defeated with either a thrown Koopa's shell or a Ground Pound from above. You will get a coin or an item if you ground pound it.
