This is not just a problem on Mercedes cars. Almost every car manufacturer uses foam to seal flaps and edges of heater and evaporator.
I usually ignore it because there isn't that much foam in there. I'm not sure how long you've had your problem, but it should be temporary. Afterwards you might get one or two pieces and then it's all gone.
You won't get rid of it by just cleaning the re-circulating flap. To me, taking the system apart to clean the foam isn't worth it. Anyone that ever did an evaporator job would have to agree with me.