See the latest discussion on brake deposit on the RB Forums.
If your brake rotor has a deposit like this:
Use steel wool and brake cleaner or a Brillo pad to scrub off the buildup.
Since the deposit was emitted from the brake pads at high temperatures,
it may be difficult to remove the deposit without heating up the rotor. In this case, you
will have to heat up the rotor surface with a torch and scrub with coarser steel wool.