It's calcium, and vinegar will only make it worse.

White scale on pool tiles is calcium carbonate from hard water. Vinegar is acidic and will etch the tile grout and maybe even the surface if you have stone or plaster. Use a pumice stone or a dedicated calcium remover.

The buildup happens when pool water has high calcium hardness and pH drifts up. As water evaporates, calcium is left behind on the tiles. Scrubbing with a pumice stone (wet, lightly) works on glazed ceramic tiles. For glass tiles or milder deposits, a non-acidic tile cleaner with a stiff nylon brush is safer.

If it’s really baked on, you need a professional-grade calcium remover like a sulfamic acid cleaner. Follow the instructions exactly — too strong and you’ll damage the grout. And for the love of god, keep the pool chemistry balanced. A calcium hardness level around 200-400 ppm and pH at 7.4-7.6 will stop new scale from forming.

Future You will thank Future You for checking your water balance.