People can easily confuse 汗斑 (fungal infection caused by fungus) with 濕疹(eczema - hypersensivity of skin). You may need to confirm the skin problem before starting treatment/ management because there are different approaches involved.
Fungal infection usually occurs in areas where sweat a lot such as feet, groin, and skin folds. The appreance is whitish in color and pattern similar to a ring. The general approach is to keep the affected area dry, ventilated, and wear cotton material. Turn on the air condition if the room atmosphere is moist and warm/hot - moist and hot environment can trigger fungal infection. If doesn't work, try to use non-perfume baby powder or calamine lotion to control the infection. If still doesn't work, probably need to buy antifungal cream from pharmacy la~~
Hope the information works.