The Green Phantom appears in Ghostbusters and in game mechanics, he is used as the source of power-ups. Shooting the Green Phantom will release a ball that will replenish the player's life or weapon energy. The resource replenished can be determined by the color of the little blob he drops upon dying: red will increase your energy level and blue raises your health. It is possible to catch the blob by jumping for it, but if you fall too far after zapping the Phantom, you won't get your power-up blob.
Touching the Green Phantom inflicts damage. He has been used as a sort of trap in some parts of certain levels - for instance the second level and the Castle level - with Loot safes as bait. In order to get to the safe, you must first squash this guy without getting zapped in the process.
It is quite noticeable that the Green Phantom looks just like Slimer.