From the Super Mario Wiki
Ceiling Surprise is a smiling white ghost that hides on the ceiling, and drops down to startle Luigi in Luigi's Mansion. It is generally coupled with another common ghost, usually Bowling Ghosts. Ceiling Surprises are extremely weak with zero HP, and are guaranteed to always drop a ten- or twenty-HP restoring Heart when captured, provided they are not defeated via elements. It also goes away when sprayed by an element. Ceiling Surprises will keep returning in Area 2 and 3 until the area is complete. They have a variant called Purple Bombers. Ceiling Surprises have an attack strategy similar to some Creepers from Luigi's Mansion: Dark Moon. One of them appears in the well and must be captured to get the key to the Rec Room. They are the weakest ghosts in the entire game, as they have 0 HP, and have no way of damaging Luigi.