Thomson brakes use a series of brake discs to stop your load. These friction discs absorb the torque in your system and turn it into heat, which is then dissipated to the surrounding air. This energy transformation via friction naturally creates wear on the discs, similar to wear on a car's brakes. This wear is generating the dust.
The dust is normal and expected, however, volume will vary depending on the environment and use. The dust will not affect the life rating of your brake. Braking a system in motion (dynamic braking) will generate more wear and, thus more dust, than braking a system at rest (static braking).