I think I decided that it was too complicated to be an ooze.
The silver slime is an artificial life form created by the Arcane, and it has a three phase life cycle: colony of amoeba-like blobs; slug-like aggregate creature called a "plasmodium" and then a stalked tree-like structure with fruiting bulbs. In the ecology section it is delightfully described as "a weird collective life form [that] functions as scavenger, repairman, and air freshener".
I think that given their similarity to slime molds and other fungi, plant is probably the best D&D fit. Even though, of course, fungi aren't really plants at all.