These are calcium sulfate (gypsum) crystals. Would you believe we found them "growing" in a pool? The well water supplying the home is very high in both sulfate and carbonate.
As the saturation level for these chemicals is exceeded, the crystals grow. They are primarily on the tile, but also on plastics and plaster. The pool is an awsome sight
- it glitters in the sunlight - but it isn't something you would want to get in!