I'm afraid you can't just "clean" it off...

It is actually the rubber material itself chemically breaking down and disintegrating. I've had exactly the same thing happen with the radio volume control buttons on both my 1999 E300TD and 1994 C280. I've since replaced the buttons on both radios.

If they are available as parts separate from buying the whole steering wheel itself, you'll just have to buy new buttons.

J.G.