Ding! You're seeing precipitation. The water which you had mixed was way off in chemistry from the water which was in your tank (Alk, Ca, Mg). I'm willing to bet the Alk was 2+ times what you have the tank at, and therefore the pH was very elevated. Combine that with your existing tank water, and precipitation happens. You're looking at dissolved limestone floating throughout your tank. In the future, you'll want to do smaller water changes (10-15% of the system's volume). You'll also want to start testing some key parameters of the new water v. your tank's current water before you do a water change, and make adjustments if necessary (Salinity, pH, Temp, Alk, Ca).