It's caused by the pH getting very high and the calcium coming out of suspension in the water. The sand paper feel is calcium sticking to the sides of the water. Whatever you do, don't drain the spa because it will harden and be very hard to remove. Best way to remove it is to get pH down and get the pH really low in the spa. That will soften it up and allow it to be wiped off with a little bit of scrubbing. Mr Clean Magic Erasers work well for removing the scale.
