This is small pieces of excess open cell foam lining that is located inbetween duct connections to prevent air loss and on the backs of some vent flaps to quiet movement. With age and temperature fluctuations, the glue that holds this foam on deteriorates and the foam falls off. The air flow in the ductwork makes it find its way out at the vents.
The fact that it only happens on your car during heat/defrost indicates it is most likely just the blend or mode door insulation that is coming off.
There is no urgent need to repair this unless the occassional debris coming out bothers you. Considering the cost is typically fairly high to take the dash apart on most vehicles, almost everyone just waits until all the loose particles are gone, then it stops :)
ASE Certified Technician
16 years automotive and OTR repair including specialized training from Toyota and Mitsubishi