The tiles in my fountain/pool turned white!! Help!!!

This is a calcium and mineral build-up from hard water deposits, same as appears on your faucets and tea kettle if you live in an area where this is common ...(AKA So.California).

Although it looks awful, it does not hurt the tile.  You can try some elbow grease along with:

Product #1: http://www.oxyboost.com/productPages/mineralMagic.html
Product  #2: http://www.intlstoneworks.com/specialty/mdr_instructions.htm
Product  #3: http://cmcchemical.com/featured/210_mineral_deposit_remover.html

Product #4: http://www.scumoff.com/ 

With or without any of these products, you can use a single edge razor blade scraper with handle and, after applying a soapy film to lubricate the surface, lightly scrape the worst areas with the blade flat against the surface at about a 45 degree angle to the surface in the direction of the push.
 

Then use something like a 3M plastic scrub pad (usually green) and an everyday scouring product such as "Bar Keepers Friend" or a "soft scrub" scouring powder . Again, don't scrub with a lot of pressure, just use a lot of back and forth scrubbing with light pressure and keep checking the surface by wiping it clean with a sponge and dry with a towel to see if the film is gone.