Martian dust is extremely small and fine-grained. Atmospheric dust on Mars is approximately three microns in diameter, and adheres via electrostatic forces. You can't just "brush it off" like you would on Earth; much dust would remain.
Is this charge mostly negative? Mostly positive? Roughly equal amounts of each? Dependent on the triboelectricity of the surface it rubs against?
(Glass is one of the most positively triboelectric substances, and covers most solar panels. You can blame glass for the "wrong" direction of the charge convention. Ben Franklin was given a glass rod to conduct experiments on electricity, directly resulting in his choice of which charge is positive. Had he been sent a sulfur rod instead, the convention for current would be in the same direction as electron flow.)