Efflorescence is a white powder deposit on face of exterior porous tile caused by water intrusion under tile trying to escape that has been sealed.

Efflorescence is a White Powdery Deposit Caused by Water Intrusion Under Tile Installed & Sealed on the Exterior. The white spots you see on the Mexican tile in the before photo above is called Efflorescence which is caused by water getting under the Mexica/Saltillo porous tile via a flood or constant water intrusion, like nearby sprinklers at a patio. 

The source of the water must be diverted or shut down. Efflorescence is a white deposit (usually calcium carbonate) that develops on the surface of grout and/or tiles as they dry out. It happens when soluble, naturally occurring mineral salts contained in Portland cement, or materials used in conjunction with it, rise to the surface.

Efflorescence is a similar phenomenon. As cement or cement tiles dry, the lime in the cement rises to the surface with the water and reacts with carbon dioxide in the air to form calcium carbonate. In this case, a white powder or film forms on the tile's face. Efflorescence has been a problem for many years, and is a topic of much controversy. The formation of these salt deposits are not mysteries. 

Efflorescence, for the most part, water-soluble salts that come from many possible sources to mar and detract from an other­wise beautiful and serviceable structure. First of all, there must be water present to dissolve and trans­port the salts, remove the source of water and your almost there.