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.
The Boo Ball returns in Luigi's Mansion 3, appearing in the Sandy Grand Hall in the Tomb Suites on the tenth floor. Unlike in Luigi's Mansion, the Boo Ball cannot be destroyed by sucking it in and then blowing it out. Instead, the player has to fire it at the top of the pyramid in order to destroy it. When the Boo Ball is destroyed, it will release three Gold Bats and the player will earn an achievement called "Found and popped the beach ball in Tomb Suites".