This is likely the foam gasket material used in the HVAC system to seal the various "flap" type doors that open and close to divert blower air from your feet to your dash vents to your defrost vents. Not much you can do short of opening up the system. The good news is that at some point it will stop... the bad news is that the doors in your HVAC system will no longer have gaskets to seal against.
There are a couple of other place it may be coming from. I would pull the filter and housing and see how it looks. The filter housing is under the bonnet on the pax side.
Pelican Technical Article: Boxster Air Filter / Pollen Filter Replacement - 986 / 987
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