Efflorescence will grow out of the grout and will leave a chalky residue on your fingers if you wipe it.

If the installer did not use distilled water when mixing the grout, what you're seeing are the minerals in the water that have gone to the surface of the grout when it was curing. Very common especially if you have hard water.
Use a flat head screw driver or a knife to etch the surface of the grout, scraping away at the top layer. You will see the true color of the grout below this top layer.
Also please note, if the installer used too much water when cleaning the tile after grouting, this too call pull the color pigment from the grout causing it to look very dull.