I have soft water too, and my apple snails always have bba growing from the pits in their shells. I've found that if I paint them with clear nail polish (preferable before they start pitting), it prevents shell erosion and helps repel the algae.
It's not just soft water or after erosion. I have some rams in extremely hard somewhat alkaline water and the biggest ones have a very impressive coat of *fur*. I think it's neat. Only happens in that one tank.