Ectoplasm is what ghosts are supposed to be made of.
What's New Scooby-Doo?
When Daphne Blake found a sticky gray substance all over her new shoes, while investigating the Leland Family Cemetery, the gang took it to Crawdad Mike, who told them it was ectoplasm, but in reality, it was residue made from the dry ice he was using when the holographic projections of the Ghosts of the Leland Brothers appeared, which he also created.
Scooby-Doo! Mystery Incorporated
It was among the list of ingredients that Scooby-Doo and Professor Pericles needed to cure the citizens of Crystal Cove from Aphrodite's love spell. It was a green slime (referred to as ghost mucus by Pericles) found at the Crystal Cove Spook Museum. It was highly unlikely that there was any real ectoplasm in the museum. Most likely this was some slimy stuff sold to tourists as "real ectoplasm." In part, this was a ploy so that Professor Pericles could get an industrial drill bit for his other agenda.