This is a sessile, sap-sucking insect, most likely a whitefly larva close to maturation. Like you say, they attach to the underside of leaves and don't move. Unlike some scale insects, to which whiteflies are related, these will eventually hatch into a more normal-looking fly. Here's an image of a whitefly larva for comparison:
![Late fourth instar whitefly larva](
The second image may be the abandoned skin after molting, these are usually more visible than live larva. More photos showing more of the life-cycle and different species are available on this University of Florida website.