I had this happen on the Slider caps on the Korg Oasys a few years back the rubber breaks down over time and it’s like the material is melting and they end up being tacky and sticky to touch.
I’ve also had this happen on an SE electronic condenser mic as it has a rubber coating,it’s ended up like tar!!!
I don’t think there is anything you can do or use specific products to clean it,it’s just a natural breakdown of the rubber.
If it was me,if they are getting worse would just take the panels off and cover them with a custom vinyl skin wrap that way at least you know it’s not going to be a dust/Finger print magnet and vinyl cover will wipe clean easier,if it ends up anything like those knobs caps on the oasys it becomes a nightmare







