Red mystery Dust
I had seen this "dust" before on previous 190E's I've worked on. That dust apparently comes from the bronze bushings on the blower motor's shaft. The bushings are composed of copper and tin. Thus the reddish color. When they begin to wear out, the dust flies and the blower usually starts to growl or scream like a banshee when its operated cold. If there's lots of dust, I'd say your blower's days are numbered. Any free play in those bearings allows the shaft to vibrate and that eats up the carbon brushes that powers those fans.