They are some form of super-deities, or perhaps a personification of some multiversal axiom about true evil. They are never described in character/deity terms, and in fact have imprisoned some impressively powerful beings, including a couple deities from other D&D game worlds. (Vecna, specifically, being the most noted, but there was another.)
The Dark Powers are noted only by what they do, and that is, they reward evil acts with more power, but at a price of less freedom. Sufficiently corrupt individuals find a chunk of the demiplane altered to become their own personal hell... LOADS of power, but trapped forever there, and unable to get whatever it is they truly desired in life.
The Dark Powers also search the multiverse (or at least the Prime Material Plane) for certain "worthy" individuals, and set the Mists upon them. If they don't flee the Mists, anyone near the victim also is transported. The area is removed from where it was, and attached somewhere in the demiplane.
It is worth noting that they control the demiplane, and can affect all within it. They can not, however, prevent Vecna from granting spells nor communing with his clerics, but they can prevent him from making personal appearances. They have moved regions about, deleted regions, and made and deleted borders on an apparent whim.
The Dark Powers do not use manifestation forms, do not reveal themselves if they use mortal forms, do not grant spells, are not subject to commune spells. Even calling them entities or an entity is a stretch.
Source Note: It is worth noting that the nature of the Mists grabbing parts of the Prime Plane was the focus of one of the TSR Retail Play modules for AD&D 2E; in that same module, PC's who do not flee the Mists wind up in Ravenloft as well. The demi-plane does not provide inhabitants; it captures them with the evil being.
In the 3E materials from S&S, it's noted that at least one domain dark-lord was originally from the Demi-plane. Another was in the demi-plane already when the Dark Powers noticed them.
This answer is not RAW, but RAW probably does not answer your question.
In the real world, slime mold moves, but very slowly. With regard to combat, they are functionally immobile. With regard to life cycle, it's entirely possible they could climb back up to the ceiling in a few hours.
Alternatively, it could be part of its life cycle. Green slime may need to consume an animal in order to reproduce. They collapse onto an animal, kill it, then use the abundant nutrients to form fruiting bodies. These release spores into the air, which attach to the walls and ceiling and form more green slime. The slime on the ground dies, its purpose fulfilled. You can basically think about salmon swimming upstream, spawning, then dying, except instead of a stream, it's a person.
Best Answer
Usually — and across multiple editions — creatures large enough to swallow other creatures whole deal acid damage to those swallowed victims. For example, Dire Shark in 3.5E or Giant Frog in 5E, or Behir in both of those editions (3.5E, 5E).
Since, as you note, this prevents regeneration (again, across editions), I think this solves your problem, at least for creatures with similar biology. Its easy to extrapolate from this that, in general, being in a stomach does acid damage, even if it's on a smaller scale for creatures who have to chew their food first.
For weird elemental or aberrant monsters with some other kind of digestion, I think it'll depend on the specifics of how exactly that biology works and the mechanics of troll regeneration in the edition you are using — and on your world.