Minerals when draining water heater
This is completely normal and is the reason it is suggested you drain and flush your tank periodically.   Even better than just draining a gallon of water is to turn off the tank (either kill the electricity or turn off the gas valve), shutoff the water to the tank, drain the entire tank, and then with the drain open, run the water to flush any remaining sediment away.

The discharge will be any colour from white to black depending on the mineral contents of your water.

Be careful when draining the tank as the water will be very hot.

Once complete, re-fill the tank completely and then turn on the power or re-light the pilot and turn the gas valve on.