Stanchion guard or lower guard/cover. Look first to the maker of your fork. They should sell spares. If you can't get it, look to an aftermarket MX plastics company like Acerbis. Ones for something small like a KX80 may work. If you are desparate, fashion your own from some large diameter flexible plastic hose. Good luck.

Is that a Maverick fork? If so, contact Maverick American.