Calcium scale
Also called: scale, calcium buildup, calcium carbonate scale.
Calcium scale is calcium carbonate coming out of oversaturated water and hardening into a white-to-grey crust that won't wipe off, on tile, at the waterline, on salt cell plates or inside a heater. It is what a positive saturation index (LSI or CSI) looks like once it lands on something.
Why it matters
Four things push water there: high pH, high calcium hardness, high total alkalinity and warm water. A drop of muriatic acid fizzes on the common carbonate kind; if it doesn't fizz, it's likely calcium silicate, a harder grey scale from very high pH. The fix is balance, not a bottle: lower pH first, the biggest, fastest lever, and keep pH around 7.6–7.8 without letting it climb past 8. Only draining and refilling lowers calcium. A wet pumice stone works on hard tile, never on glass tile, vinyl, fiberglass or painted surfaces. In a salt pool, scale plates onto the cell first.
In numbers
If calcium hardness is very high (roughly 500–600+), a partial drain-and-refill is the only thing that actually removes calcium. Balancing alone can't fix water that's simply too hard.