From the Super Mario Wiki
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Wario about to hit a Face Block
. A Face Block that has already been hit can be seen to the left.
A Face Block is a block in the game Wario Land: Super Mario Land 3. Much like its name suggests, the block has a face on it that is quite cheerful. When Wario hits one of these blocks by jumping, a coin, heart, or a power-up may appear. These power-ups can be one of the following: Garlic, a Bull Pot, a Dragon Pot, or a Jet Pot. When the block has been hit, it's happy face is turned into a much more grumpy, angry one. The blocks are indestructible. Wario can also get the treasure inside by launching Dragon Wario's flame at it.
These blocks are similar to the ? Blocks of the Mario games.