Sticking metal dust on a charging case AirPods

The charging case likely is affected by magnetic fields at some stage with perhaps some residual magnetism being present which would attract iron dust. Those look like iron filings to me so for me the big question would be where is it coming from?

Here is the standard Apple answer for cleaning Airpods: How to clean your AirPods, AirPods Pro, AirPods Max, and EarPods - Apple Support

If it is literally sticking to the case by magnetism I would contact Apple and ask what is going on. If it is something in the Airpods that is degrading then they should address the issue. If it coming form something in your environment then you may have to see if you can somehow pinch the dust and remove it because it won't just rub off if it is magnetized. Well it might if you took a slightly damp cloth and could drag it away from what it is sticking to into an area away from the magnetized zone.

Contact Apple for support and service --> "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232

AirPods Service and Repair

Learn more about AirPods service options, warranty, and pricing. - https://support.apple.com/airpods/repair/service

Apple One (1) Year Limited Warranty – Accessory - For Apple and Beats Branded Products Only. - https://www.apple.com/legal/warranty/products/accessory-warranty-english.html

This will happen if you are anywhere close to environments with lots of metal and/or metal works. I work in the rail way industry and my AirPods will look similar even if I do not take them out of my pocket at work.

I use a combination of Q Tips and tooth picks to clean the filings off the magnets. The case will get permanently discoloured wear the magnets meet as the metal filings are ground into the plastic, not much you can do about this.