Hello. New to the forum here. I'm unexperienced with pools for the most part, but have been setting tile for many years. I have seen the same buildup in shower floors when lots of lime was used in the final setting bed, I've even seen a drain almost completely closed off! Muriatic acid will clean it up nice, but continues to eat away at the grout and eventually whatever was used to affix the tiles to the pool. Something to possibly check into would be epoxy grout. Now I'm not sure if it is pool friendly, but it's pretty much inert, and wont have the leaching issues that cement based grouts do. I would have to sway against it being calcium hungry water in your pool, you guys are on top of that. With that said, removing grout can be time consuming depending on the size of the tiles, the grout spacing, and the amount of tile used.
I hope that helped and best of luck.
Aaron
** The old epoxy grouts were a major PITA to use, but the new ones like Spectra-loc (comes in a kit) are easy to use if you follow the directions.