"Never heard of it..." ...sigh.

This is a common problem with the grippy paint that URC used on earlier units... it kind of degrades over time (like how some soft rubbers/plastics self-dissolve), even without use (I have unused Sidekicks that are gummy). However, it mainly affected the bluish color used on MX-500 and MX-700 units, and I haven't seen anything like that happen on my MX-850 which is still perfectly fine.

So, if it's just accumulated grime, wash it with a wet cloth (and if you need more, try the advice given by the previous poster). If that doesn't do it and the issue is with the actual paint, then the only real cure is to try to peel the paint off. You'll lose the tacky texture, but at this point, that may be preferable.