Reef Builders - The Source for Reef Keeping Information

I am new to the salt water world. i set up a tank and most is going well, levels of ammonia and nitrate are minmal and salinity looks good on a hydrometer. I visited a store and bought live rock and live sand, about three days later I am noticing a white cloudy substance sitting on the bottom next to a large rock where i buried a large piece of frozen brine, is this normal or the beggining of a problem?