Yeah, that definitely looks like some kind of saturation-dehydration product, probably with a pretty significant acidity quotient.

But I'm no chemist; I'm just guessing from some of the gunk I've dealt with in my time.

I eyeballed the refilled saucer this morning, but no signs of infestation. It must have been the addition of water. (I guess that guy was using a pool of scented oil? Either that, or it's because he was displaying the rocks in glass containers, not metal?)

I'm still puzzled why the rocks (I really should call them "crystals") changed color, though.

I decided to google "rock potpourri" and discovered it's made from either fancy sea salt crystals or water softener crystals and colored and scented. They say to use oils to carry the color and scent (various essential oils are recommended) and to "recharge" the crystals when the scent fades.

Being salt kind of explains the odd crystal growth when you put water on it :)