Ectoplasm

Ectoplasm (also known as Ghost Slime and Ectoplasmic Residue) is a substance that comes from ghosts and out-worldly beings. It usually appears in a form of a slimy green substance, but sometimes is visually completely absent, leaving only energetic traces. All entities, including ghosts, demons, phantoms, spectres, spooks, ghouls, and poltergeists, leave behind ectoplasm when they make contact with something or someone in the physical world. Even those that can maintain full corporeal form but they tend not to leave behind enough ectoplasm to be visible to the naked eye. Ectoplasm doesn't easily launder out of clothing or hair and is known to cause gastrointestinal distress if consumed.