lol. My wedge shaped Mouse I just got had some minor black smudges on the top. That came right off when I used the rough side of my dish sponge. Just moistened with water and used light scrubbing. Came right off and it's clean with no damage to the plastic. The sponge I used is not strong enough to damage it unless I really really tried to.

I did this with it dis-assembled of coarse. I didn't get any water on the inside of it, so I just wiped it try and put it back together. But I guess some mice are dirtier then others, so might not be as easy for you as it was for me.