Bob is probably right. The evaporator has had time to dry and the dirt on the coil has come loose. You should be able to clean the coil first with a soft brush and vacuum cleaner. Then use a mild coil cleaner on it. The only way to get it out of the duct is to blow it out.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7