If it's in the tank with water and there's any source of decaying organic matter (uneaten food/poop) then nitrifying bacteria will colonize on the surfaces. That's all LR
really is, a home for bacteria to grow.
Critters usually means copepods and amphipods. The best time to look for them is about an hour after lights out in the tank. If you have a red flashlight or filter for one, use that to look around and check for any movement about the rocks and substrate. I pretty much need to use my reading glasses these days to see most of them.