Doug,
I do not know which model of thruster you exactly have, however googling "Side-Power retractable thruster" gave an image like this:
The grey towering-thing is the brushed DC motor. I don't think having carbon dust outside the motor housing is a normal occurrence unless there is an underlying issue.
I just recently finished a complete (I mean down to the bare wire) restoration of my Vetus BOW16024D thruster and I can share what the brush "retainer ring" looks like, please see the images attached.
Carbon dust can be a sign of excessive arcing due to worn out brushes or damage to the commutator. One of the issues in my thruster, for example, was corrosion of the inner wall of the brush-guide: the rectangular metal tube in which the brush slides. This caused the brushes to seize in their housing and not make contact with the commutator. Easy fix - I cleaned the brush-guide walls.
As there could be many underlying issues, your motor should be inspected. This is not an expensive job, you can remove the motor yourself, take it to an inland(!) shop with reasonable prices.