Thats your ITB's mate. Foam rollers are great for that. Maybe "the stick". Get a bike fit done, correct pedeling, maybe your feet are too pointed inward. See an osteo or chiro or physio. Maybe your hamstrings are weak compared to your quads, inner quadracep (vestus medius) stregth may be prescribed, maybe multi tracking, maybe lack of stretching, no 5-10 minute cool down, too much too soon, maybe a sharp increase too quick.
Pro bike fits are great but dont forget that if you have a current knee issue lowering the seat may help. Stretch all leg muscles before and after your rides and when not riding. Rest, ice.