Normal wear and tear of plant leaves, could be from a passing insect a random twig that fell on the leaves during a rain storm, any number of things but nothing looks amiss or troubling with the condition of the leaves.

Even the light white spots?

They look like minor damage to the leaf cell walls. Do they cover the whole plant or just a few leaves?

On this plant all leaves but on other pepper seedlings one or two leaves

I'd try to spray em with one of the bug Killer or repellent stuff for the garden. Just make sure when it's safe and when it's time to stop before getting any fruit off em per the label maybe. Worth a shot if they're eating your stuff up.