This is about a type of creature and not about the ghost character Slimer.
Ghostbusters Activision Video GameEdit
During the development of the cartoon series The Real Ghostbusters, the glutton ghost from the first film that slimed Peter at the hotel was given the name "Slimer". Even though the cartoon is not canon with the film, by the second film the character name was too well known to be ignored. So in Ghostbusters II, the green ghost returned and was dubbed "Slimer" like the cartoon series. All additional media and merchandise also follows the revision in canon.