Common garden mint
Sounds like powdery mildew. It is a fungal disease and mint is susceptible to it. Pick off all infected leaves and improve conditions.Your growing conditions may need to be changed. Powdery mild often occurs when a plant dries out - can happen easily in a pot...maybe consider have burying the pot in the ground or putting a saucer under the pot to stop this happening. If the plant is in a shady position....consider moving it to a sunnier spot. Make sure there is good air flow around the plant.