You can clean your scale using a lint-free cloth dampened with water. If water is not enough to clean it, you can also use crystal vinegar.
Do not use abrasive cleaners, soap or any other cleaning products.
https://support.withings.com/hc/zh-cn/sections/6212668113809-Wireless-Scale
https://support.withings.com/hc/zh-cn/sections/200335727-Maintenance