From the Super Mario Wiki
Blockhoppers (also known as Coin Imposters) are robotic enemies encountered in the game New Super Mario Bros.. They are found only on World 2. They disguise themselves as a ? Block, with Brick Blocks on top of them, but once Mario approaches, they reveal their true form. They are actually blue robot-like enemies that hops at Mario in cue with the background music. They can be told apart from ? Blocks by the fact that the question mark on them does not move. They are similar to the Pile Driver Micro-Goombas that disguise themselves as blocks from Super Mario Bros. 3. They can be defeated with either a thrown Koopa's shell, a Ground Pound, or by running into them using the Shell Dash ability using the Blue Shell. Mario gets a coin or an item if he Ground Pounds the ? Block on the Blockhopper.
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