Its brown dust, that usually, if i wash my wheels once a week, comes off really easily. Problem is this time I left it on to bake too long.
My Formula Ferodo pads are just way too dusty for me, maybe on par with Ultimates, but they give off a brownish dust which is the adhesive compound in the pad material moreso than metal filings.
I have given up on finding performance pads with low dust, i think its like asking for good tyres that last a long time.