From the Super Mario Wiki
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A Ghost Block is a very rare, possessed ? Block that appears in New Super Mario Bros. Wii. Only 4 of them appear in the secret area in World 7-. They guard a Star Coin. They resemble a normal ? Block, but as Mario approaches, they turn purple as they float over the character and throw themselves at him (a very similar mechanism is used by a Ghost Vase), breaking apart and releasing a coin if they hit the floor or a platform.