The part of the air conditioner on the roof is (or at least should be) separated from the ductwork side by a solid barrier, of course with some wires and tubes going through it. Outside air and inside air do not mix by design. If you think the source might be the air conditioner itself, it would be better to remove the inside cover/louver on the air conditioner unit and examine things from the inside. (There's no ducting as such on the outside side part of the air conditioner, just the shroud over the condenser, its fan, and the other external bits.)
Unless the powder is appearing from that air conditioner unit itself, I'd tend to suspect a source near where the pile is forming instead; but there's certainly no problem with taking a look at everything. If nothing else, it's well worth looking at the outside part of the air conditioner and cleaning out dirt or debris if needed to keep it working efficiently and effectively.
