My mother has a plastic bathtub insert in her home and doesn't use it at all. However, there seems to be this white powder that accumulates all over the surface of the tub. My mom will clean the tub and the next day the powder starts to come back again. I had suggested maybe it was just residue from the soap, but it isn't. The powder isn't on any of the other surfaces in the bathroom. Its almost like drywall dust.

Any ideas on what this could be?