odd, never seen this, and the bottom is sapphire/ceramic ... I'd hold it under water and use a little (dish) soap to try to clean it ... does your watch come in contact with anything other than your skin and the puck? do you lay it flat down at times?
Actually once when I put it one I slid it over my Oura ring and it did touch the bottom now that I think about it . I don’t feel any scratch intentions , I’ll try the dish soap after work . Maybe some how I can get it off ……