It's a virus. You can use a liquid copper fungicide spray to get rid of it--for now. It almost always comes back, usually at the end of your season.
I have a tree next to where I grow my peppers that shows up with it every year towards the end of the growing season. It just takes one leaf to fall off and land in a pepper plant for it to spread. If your season is about over I wouldn't worry about it--I don't. If you have a lot of time left in your season then find some copper fungicide spray and go to town. Good luck.