This is probably Tinea Versicolor or mild fungal infection. A good anti-fungal cream like Canestine should sort it for you.
Yeast, thrush, jock it, it's all the same thing. Treat with an antifungal cream like Lotrimin as directed on the tube. Wash socks and underwear in hot water with measured Tide and about 3/4 cup baking soda to kill the yeast.