Ectoplasm

Ectoplasm is a tar-like substance that only appears when encountering one really pissed off ghost. Meaning, not just any ghost can make this stuff. The ghost has to be seriously angry for this stuff to appear. It also gives off high EMF.

The Winchesters have seen this stuff before once or twice. This proves that this supernatural phenomena is rare. It first appears in the show in Season 2, No Exit.

No Exit
Sam and Dean are investigating a case where women in an apartment are going missing. While scaning the room, Sam picks up high EMF signals from the light switch, and shows Dean. Dean touches it and shows Sam. Sam comfirms it to be Ectoplasm. Later on, when Jo is by herself in the walls, ectoplasm seeping through the walls before he gets grabbed by the ghost.

After School Special
Ectoplasm comes from possessed students who are possessed by the ghost of Dirk. It came out of their ears or noses.