It is certainly a reaction to the self-cleaning of the cell. The reversal of polarity can remove the scale on titanium plates. The reversal time may be inadequate, the thickness of the scale is too important to be completely dissolves during the self-cleaning. Acid created by the device will dissolve the calcium of the coating. Then the water flow will carry the scale to the backflow. Reducing the reversal time coupled with acid injection of your regulator should gradually decrease the appearance of these plates.