billc64, my opinion is that they are mealy bugs or similar soft scale because:
They concentrate along that main leaf-vein and in the angle where two veins meet.
"A couple of different pepper plants in my garden have yellowing leaves that are dropping off." This can be expected when a leaf-sucking insect infestation runs unchecked. link

YOur link works fine. Thanks for doing that.

Can the whatevers be easily removed?

If not, I wonder if it's a raging case of edema. If so, that's a physiological problem caused by the growing conditions of high humidity & moderate temperatures.

That said, edema doesn't usually cause leaf drop.

It might be helpful if you post 2 more images:
1. A close up of the top side of an affected leaf.
2. The plant overall, or a very large portion of it.

Just to be thorough, is it dirt - splattering up from the bed to the bottom of the leaves? If you've had heavy rains, that could splash the dirt up to the bottom of the leaves, and make the plant waterlogged to where the leaves turn yellow as well. Otherwise, scale comes to mind.

I agree that a couple more pictures would help (as indicated above).