If it looks like it has been shredded it most likely have come through the return air duct of the air handler and it was shredded by the blower. What is it… I’m not sure, spongy appearance would suggest AC refrigerate line insulation or insulation from the interior walls of the air handler. In case your blower is belt driven this material could be from the belt itself… I don’t know how much of it there is.
There is a possibility that something was inside the air duct and it was sucked into the blower compartment… If you have a drip pan underneath the air handler use the coil cleaner (search for “ac coil cleaner”) and rinse the coil with water after using the cleaner (unless otherwise specified in the cleaner’s manual). You may want to put some plastic foil underneath to prevent ceiling damage below.