If it`s recent yellow road stripping/marking paint that splashed up while it was wet from your driving over it, you MIGHT be able to remove it by using an adhesive remover OR Goof-Off (NOT Goo-Gone) on a microfiber. If it`s paint transfer from a light scrape against a post like that at a gas station to protect the pumps, then sanding/polishing is your best recourse. It sounds like the former, since you mentioned the mud flaps. If it`s been there awhile and has cured, it will be difficult to remove. I assume your mud-flaps are plastic, so any solvent used to remove paint could also discolor your plastic. Just a warning.
Someone might suggest using a plastic razor blade to scrape it off. I think you may induce serious scuffing and scratching that will be too difficult to remove unless you have surgeon-like hands and ability. What ever you do, don`t use a metal scraper or metal razor-blade holder, despite being tempted to do so. Then again, it`s your vehicle.