Today was "watering day" so I seized the opportunity to take them out and inspect it more closely. Those white specks (you have good eyes, I didn't see them myself at first) do not look like much to me, I scraped some of with my finger pad and looked at it under the magnifying glass, it doesn't look like an insect, it could be some sort of droping but I didn't see anything that moved anywhere.
I'm thinking it could be mineral deposits.
I could find my old microscope from its dust and look at them more closely then, should I?
I also noticed that the ventricosas are the only ones affected, maxima, sanguinea and coccinea, all of them in the same tank with the ventricosas, show no sign of that rust at all.