The High TDS alert will be displayed when the TDS of the water is above 2500ppm, the fault light will appear when levels are above 3000ppm.
The alert is intended to advise that you're not swimming in the absolute freshest possible water. High TDS also means you may not be getting the best cost effectiveness through power consumption and chemical levels.
Try diluting some of the pool water. TDS levels should never exceed 3000ppm.