Ectoplasm is a tar-like substance that only appears when encountering a very active and angry ghost. The ghost has to be very angry for ectoplasm to appear. It also gives off high EMF. The Winchesters have seen ectoplasm before the events of No Exit, only once or twice. This proves that this supernatural phenomenon is rare.
There are two known types of ectoplasm, the first being the common black colored ectoplasm given off by extremely angry ghosts. The second one is pretty much similar in consistency, but its color is more akin to moss, and it is given off by a type of ghost called a specter.
In No Exit, Sam and Dean are investigating a case where women in an apartment building are going missing. While scanning the room, Sam picks up high EMF signals from the light switch, and indicates it to Dean. Dean touches it and shows the material on it to Sam, who confirms it to be ectoplasm. Later on, when Jo Harvelle is by herself in the walls, ectoplasm is seeping through the walls before she gets grabbed by the ghost.
In Southern Comfort , ectoplasm of the green type manifests multiple times. At least one sample of green ectoplasm is found for every host that the specter possess. The ectoplasm was usually found leaking from the host's ear.