Green Slime has been regarded by many as less of a monster and more of a hazard given hit points. But in Classic D&D it has a movement rate--extremely slow, but it does move. And more telling, it has a Morale score of 7 in Mentzer D&D*.
If it can break and run, or at least stop attacking, it's not a hazard like a pool of lava or a fountain of acid that looks like water.
*Morale is 12 in Moldvay Basic, but it can move. OD&D, Holmes and AD&D all list it as immobile, and don't list Morale scores. Is Mentzer a typo? Don't know, but I think I like the implications of Green Slime breaking morale.