what is brown "stuff" coming out of front center vent? falls apart when touched

MICE HAVE BUILT A NEST IN BLOWER HOUSING NEED TO CLEAN IT OUT. PROB. NEED MECHANIC. THIS HAPPENS A LOT TO MANY AUTOS. SEE IT OFTEN IN THE SHOP. WITH FOREIGN MATTER IN FAN BLOWER WHEEL WILL HUM AND VIBRATE WHILE PUSHING STUFF OUT HEAT/AC DUCTS.

or foam on duct connrction is starting to fall apart