From the Super Mario Wiki
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Phantom Hands are floating, disembodied gloves that point to secrets in New Super Mario Bros.. They only appear in the World 7-Ghost House, and their only purpose is to point out the location of an invisible block containing either a ? Switch or a power-up that moves back and forth to the player. They are not enemies, meaning that Mario or Luigi are not injured if they touch one, nor can they be defeated.