A little late I know, but from the pic it looks like Artillery Mold Spores, or Artillery Fungus. It's an interesting read if you Google them. Seems they are most predominant in mulch that you find beside your hour or driveway.
We start by spraying a degreaser on the effected area. Let it dwell for 5 min or so. While washing the car a top scab can easily be removed with your fingernail. The remaining mark is the more difficult to remove depending on how recently the car had been waxed and how long the fungus had been attached. As several have said, the fingernail or a plastic blade can be your best process. It becomes much easier after the spore scab is removed and the remaining spot softens up.
