I have the same problem and it started 3 years ago. I painted my in-ground kidney pool about 8-9 years ago. I bought the paint from a paint pool dealer and who recommended the paint, scraping the old paint, washing it with muriatic acid, resealing the walls ... etc.

I did some research what causes this ... everyone had their outrageous oppinion but one viable exlanation was the UV ray over the years breaks down the paint and becomes chalky.

Another one is if the pool is low in calcium, it draws the calcium away from the paint that's whys it chalking. I've been maintaing the calcium level at the correct level but it's still chalking.

My wife painted murals fish and sea corals on the surface and we are sorry to see it fade. I have not heard any suggestion on how to correct this but repaint it again which is not an easy task. I hope someone can give a suggection.