It is quite normal for little pieces fibre glass to blow out of glass pack mufflers when they are new. It is hard to see as they are very fine. You can only notice the reflecting light from it if your car has not been washed for a while, but has not gotten that dirty.
I am still noticing small amounts after two vehicles and several thousand km's. Both my cars have been jet black which makes it easier to see.

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Geoff.