The Fleeing Ghost looks like a typical ghost with one eye in the middle of their forehead, which can be found in The Ghost World. The Triangle Kerchief Effect can be obtained by interacting with them.
They move quite fast and flee from Madotsuki which is the inspiration for their fan-given name. The easiest way to catch them would be with either the Stoplight or the Cat Effect, as they show a normal reaction to both effects. They also react normally to the Knife Effect like most NPCs.