The jellies, oozes, and slimes encountered in the dark places of the earth are amorphous creatures that are the bane of all living things. They dissolve weapons, armor, flesh, and bones of their victims. They are an asexual creature and solitary beast,occasionally diving into offspring. These creatures live only to eat and reproduce.
Jellies, oozes, and slimes seep through dark corridors, cracks, and under doorways of dungeons, searching for flesh to devour. They can easily travel on walls and ceilings, dropping on unsuspecting prey.
These creatures are sometimes tolerated in underworld areas simply to act as janitor, dissolving carrion, trash, offal, and dung. However, this practicing can be dangerous and is not usually appreciated by most underworld creatures.
“My concept was that both [grey oozes and gelatinous cubes] were accidental
creations of careless wizard alchemists that dumped various failed magical and
alchemical experiments down the drain or into some cess pit. These admixtures
affected single-celled life forms, thus eventually engendering the various
jellies (and a gelatinous cube is one of those), oozes, puddings. The slimes
were generated in similar fashion, the waste affecting normal slime.”
— Gary Gygax (1938–2008), October 1, 2006, EN World Q&A XII
New Monster: Cerebral Slime (towerofthearchmage.blogspot.com)