Efflorescence is a white deposit on face of exterior tile caused by water intrusion under the tile, like from sprinklers, that has been sealed.

Efflorescence on exterior tile is a white, powdery, or crusty mineral salt deposit caused by water carrying dissolved salts from mortar or concrete up through the porous tile/grout, leaving a white residue upon evaporation, if tile is sealed it will have  white milky look. It often appears in damp environments on porous materials such as clay Saltillo/Mexican tile. Efflorescence is a White Deposit Caused by Water Intrusion Under Tile Installed and 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 Mexican tile via a flood or constant water intrusion such a outdoor sprinklers near 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 of the tile.

Efflorescence is a similar phenomenon. As cement or cement tiles or clay 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, a white dusty powder. 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 especially in South/West Florida. 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.