From the Super Mario Wiki
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A Boo Ball is an item in Luigi's Mansion. While exploring the mansion, Luigi may follow the Boo Radar's signal to a Boo, only to find one of these decoys instead. Boo Balls are nothing more than white spheres with the face of a Boo printed on them. They do not harm Luigi, and he can suck them up onto the nozzle of his Poltergust 3000 and blow them out again, destroying them.