A pumice stone will do the job if you're careful.

Yes, but it takes patience and you have to keep the tile wet.

Calcium scale is basically rock. Scrubbing with a brush is like trying to erase concrete. A pumice stone is soft enough to not scratch glazed tile but hard enough to grind down the deposits. The trick is wetting both the stone and the tile first — dry pumice will leave gray marks that are a pain to rinse off.

Work in small circles, rinse frequently, and don’t press hard. If it’s not coming off after a few passes, the deposit might be too thick. In that case, you can try a mild acid wash (muriatic acid diluted 10:1) but only if you really know what you’re doing — that stuff will eat through grout and kill plants. Pumice is safer.

For the love of pool chemistry, don’t use a metal scraper or sandpaper. You’ll scratch the glaze and then calcium will bond even harder next time.

If the pumice feels like sandpaper after a minute, throw it out and grab a fresh one. They’re cheap.