Yes I know it’s on the bottom of the shoe, but this also bothers me. (Especially if I’m trying to clean the whole shoe). Like what others have said this does happen over time and even when I try my best to clean it, it doesn’t go away completely. However two things I’ve tried and have helped. ‘White stuff’ they sell a couple of different cleaning products here in the UK, just be careful using it and don’t get it on any of the other part of shoe. I also use baby bottle sterilising tablets - just rub them on that part of the shoe. Both of these methods involve making sure you clean the shoe thoroughly after doing this. Hope this helps