Might be the stuff they put on the tires to make the beads slipery to mount them. I would try to scrub them with a good wheel cleaner first. Should come off if that's what it is. I just had both tires on my bike changed and had to clean them before I put the wheels back on. There was this funky stuff on em'.
I'm diggin' the wheels :thumbsup:
I'm diggin' the wheels :thumbsup: