From the Super Mario Wiki
A paranormal shield grants ghosts invincibility in Luigi's Mansion: Dark Moon until it gets shattered by an electric pulse from Luigi's Poltergust 5000. The paranormal shield is a certain color, indicating which level of the power surge is at least required to shatter it. In Dark Moon Quest, paranormal shields are introduced in mission Play Catch where a single Greenie in the Warehouse uses it. In ScareScraper, only the Beetle Whisperer and its variations of them use the paranormal shield in normal mode. During harder modes, other ghosts such as Greenies use it.