We don't know exactly where the boulders on top of the delta came from, we know they were washed into the crater by fast flowing rivers, so did they come a short distance or many kilometers. AFAIK the exact source of the light toned rocks is not yet understood by the science team. Most of the terrain in and around the crater is covered in dust / sand so there could be more of these lighter coloured boulders, but they are just masked from our view!

I am not a geologist, but it could be that this particular rock was well polished and that prevents the dust from accumulating on the surface, but that's a pure guess. Maybe the science team will be as curious as many of us here and command a drive over to it and take a closer look :)