Mineral salts... I don't think so.
If it where mineral salts then they should have risen to the top of your cubes. I don't see how you could get that much salt buildup from watering cubes one or two times with almost plain water. If it is salt buildup then I don't think your clones will take. Your supposed to use VERY weak nutes/no nutes until you have white/white calluses on your stems.
The wrapped cubes you are using tend to be a little too wet. After your first soak,(before you stick your clones) you may want to give each of your cubes a little squeeze.
Did it look like little white dots about the size of a pin head? If so it's a week mold or mildew.
It's either a weak powdery mildew, the paper from the sides of your cubes, whatever eats the paper that comes with your cubes (mold or beneficial), pieces of the same white plastic that the clones come in, or an inoculant that comes with the rockwool. (heard that there is an inoculant that comes with these cubes but thought it was bullsh*t)
I clone plants for clubs in california.
