The Thermasol filter is a particle filter, not purification.
White powder substance is lime/calcium. As the water boils to produce steam (distilling process), the impurities come out. Some of what comes out is lime/calcium.
A suggestion from my personal shower... I always squeegee down all walls, glass and use a dry wash cloth to wipe down the control, steam head, shower head, etc.
Or, you can look to the water treatments systems