"Oozes" is a general description for any creature made of an amorphic blob of unidentified material that is trying to kill you.
Oozes are often created by mages using the power of Green Embers by animating a blob of organic refuse. The resulting ooze will attack any other living thing that gets near it, making them cheap guardians.
Oozes can also form naturally in areas of high Ember energy and large amounts of organic material. Theorists believe this is similar to how undead will rise on their own due to Ember corruption near a burial ground.