Are you using a hand sanitizer or hand soap? Sanitizers will have a high percentage of alcohol which will remove any coating the lenses have on them. Usually sunglasses are coated with a film to make them UV protective or similar protections. From what it looks like, the outer coating is coming off, so something is degrading it

Since it looks like its just on the bottom outer rim, i'd point towards the sunscreen...Protective films are usually oils, sunscreen is an oil and like dissolves like