This is normally caused by a high concentration of Total Dissolved Solids (TDS).

TDS is an accumulation over time of all of the chemicals you've added to the water. Generally, TDS should be kept below 2,000. You can have this tested at a pool supply company.

Spas should be totally drained and refilled as recommended by the Personalized Maintenance Schedule to prevent a build up of TDS.

TREATMENT

Drain water and refill with fresh water of lower TDS. You can reduce the TDS to half by draining 1/2 of the water and refilling with fresh water.
For more details, see (Problem Solver: High Calcium/TDS)