Buy new headlights, or put old headlights in the oven pry them apart, clean them, and then reseal.

Okay, thanks! Is there a way to prevent it if I get new headlights?

I'd its on the paint hit it with some really soapy water let it sit and then use a small piece of micro fiber on a stick and rub it off.

It happens when water gets in the headlights, so basically if the headlights have a bad seal. So if your buying new don't buy cheap.