Mine does it too. I took the cvt covers off, and pulled the outer clutch cover off and checked the shoes. I then put it back together,(after blowing out the dust), and cleaned the rear sprocket (clutch) with rubbing alchohol, and it went away..... for a while. What it is, is some residue from the belt sticking on the back pulley assembly near the outer edge. It comes right off, but seems to want to return. The belt looks A-OK, so I am not too worried. I'm hoping that the belt will "wear in", as mine only has 2500 miles.
It takes about 20 minutes to clean the residue off, so it definitely isn't a big chore to do.