There are three main reasons for mint to grow white hairs, namely poor ventilation, insufficient light and overwatering, the plant is in the absence of light, ventilation and humid environment is easy to infect a variety of diseases, including white frost disease occurs when the leaves will have white hairs. When you find this situation, you must move to a ventilated light place as soon as possible to avoid deterioration. If it is serious, but also sprayed with carbendazim or use it to irrigate the root is also OK.