Diagnosis
That's carbon dust. It's normal wear material from the carbon brushes. Excessive dust means the brushes are wearing out faster than usual.
How to Fix It
Inspect and likely replace the carbon brushes. Remove the two brush caps on opposite sides of the motor housing. Use a small screwdriver to pull out the spring and brush. If the carbon piece is less than 1/4 inch long, replace both brushes (part 903371-1).
If That Didn't Work
If new brushes still produce a cloud of dust, the commutator on the armature is rough or dirty. This files down the brushes rapidly. The armature may need to be replaced or professionally turned.
Heads Up
Keep that dust away from your face. It's conductive and can cause shorts in other electronics. Regularly clean out the motor vents with compressed air to prevent internal buildup.