Black marks on AirPods max

Hi. I have started getting these black marks on my AirPods Max. I tried wiping them and it came off a little, and the next day after wearing them and putting them in the case and then taking them out they are even worse. I’m not sure what this is or what is causing it. It is not ink as they have been in my hands in the case when it happened first, and then the second time in an entirely different bag.