I've not seen an issue like that, even if you had hard water and a water softener. It shouldn't be leaving a mineral residue from hard water.

There is the possibility that is could be something like residual grout haze. But that would just be on the tile, not on the plumbing fixtures and shower door too, and it would likely show some sort of "wipe" or "smear" marks on it from the grout clean up.

It could also be one of those "first use" things.

I'd call Thermasol and ask their tech department.

Do you also sell