From the picture of underside of the leaf you either have white fly ot some kind of fungus. When you disturb the leaf do you see white flakes flying off the plant. Have you tried lightly scraping the white stuff with your fingernail to see If it's a bug. If so that's white fly. A spray with neem oil and some sticky yellow white fly cards will take care of it. White fly is attracted to yellow.

If it's not white fly then it might be mildew and a good fungicide will take care of it

spider mites maybe but i wouldnt think any pests would bother a high capsaisin plant like that. is it creating little damage spots on the top side of the leaf because that would be the dead give away for mites

I also think spider mites. I had a bunch and actually, the mildest way I dealt with them is to blast them off with some cold water. Otherwise, I think some pyrethrin type spray worked well for keeping them away. Careful though, too much is bad for your animals. It's otherwise considered safe.

ironic but i use my habanero peppers to make spray for my other plants. just like three cut up habs and a clove of garlic boiled in a pot of water and it works betterthan anything ive bought at the hydro store. just dont breathe. i do a hit and run. spray and get the hell out technique

What is this plant called? its not making any fruit or peppers