The Fleeing Ghost looks like a typical ghost with one red eye in the middle of its forehead. Its lips are puckered up, giving it a silly expression similar to someone pretending to be innocent after a misdeed. It can be found in The Ghost World, from which the Triangle Kerchief Effect can be obtained by interacting with it.
It moves quite fast and flees from Madotsuki, which is the inspiration for their fan-given name. The easiest way to catch it would be with either the Stoplight or the Cat Effect, as it is regularly susceptible to both effects. It also reacts normally to the Knife Effect like most NPCs.