Black lines on my airpod case.

Hello fellow Apple users, I have noticed that after 1 week use of my new airpods there has been appearing a black line on my case, which then with time increased. Now there are loads of them, and I literally just bought them 1-2 months ago, which means that they are pretty new. Is this some sort of a failure of the case itself? Like burnt marks, or what? I need answers, and I am really unhappy about this. This problem only occurs on the back side, of my airpod case, and not on the front which makes me think that it's maybe a battery problem? Is this replaceable?