White powder coming off your rubber (Dicor) roof is normal.
Scrubbing it down good is the only cleaning it should need. Some folks like to put a "treatment" on the roof after cleaning which can minimize the chalking.
Inspect for any tears, holes or areas of extreme aging, wherein the black base rubber starts showing thru the white top layer.
1995 Coachmen Santara 360MB 36' w/slide.
Ford F53/460 chassis, 1997 Jeep Grand Cherokee "toad"