New-to-me pool. Help with hard white deposit on fiberglass surface.

Jul 17, 2015
I purchased a house 1.5 yrs ago with an in-ground, indoor, fiberglass pool. It's about 8000 gallons and it has a SWG and solar heater. Since the house is in a rural area, about an hours drive from any civilization, it was hard to find any pool maintenance company willing to maintain the pool for me. It was especially hard to make the decision to maintain the pool myself since I don't live there, I've never owned a pool before and it's just a summer cottage for us that we don't frequent often, except for weekends in the summer. I toyed with the idea of just removing/filling-in the pool at first since it had been neglected by the previous owner for several years....but ultimately, at the request of my children, I made the decision to keep it and maintain it myself.
When I first got the property, the water wasn't green, but it was very cloudy, and when I rubbed the walls a white powdery stuff came off and made the water very cloudy and I had a hard time balancing it and getting the water clear because of this. Finally, after reading Pool School and many many posts on this site, I realized that it had a very high iron and calcium content so I decided to replace the water to get rid of the iron. I partially emptied the pool as much as I could (about 3/4 of it's volume before ground water pressure started pushing up on the bottom floor) and I refilled it with city water trucked-in from a nearby town. But before refilling, when I had emptied most of the pool water, I took that opportunity to scrub the walls with green scouring pads (took 6 of us, several days to scrub all the walls) and got A LOT of the loose white chalky stuff off the walls. Now, when I scrub the walls regularly with the pool brush, I no longer get white clouds of powder into the pool, but the walls are still light grey. After that, it took me a very long time to finally balance and get relatively clear water with the help of this site. So I have learned how to manage my first pool, which is now relatively clear and the chemistry has been balanced now for a while. I purchased the test kit that is recommended here and maintaining the pool now is no longer a daunting task.
Anyway, the issue I see now is that the pool walls, which I assume used to be a light blue color are now a light grey color below the water line. Most of the walls, above the water line, are light blue.
My question is: is there a chemical I can add to the water that will remove this off the walls and return them to their blue color, or is the pool surface just too far gone for this? I mentioned this to a pool guy that I met at the local Menard's parking lot one day and he suggested that the only way to really remove the build-up on the walls is by bead blasting it, which he said is expensive to do to the entire pool. But I'm not sure it's safe to bead blast a fiberglass surface without putting holes or pits in it.
I decided to test a small section myself and took some ultra-fine wet sand paper and scrubbed one small spot about a foot below the water line, on one of the seats; after scrubbing for a little while, I could see the light blue color again, and more white powdery stuff dissolving into the water.


(not sure how to insert a picture here).

Anyone have any experience with this build-up or know what the heck is on the walls? I know the original gel coat is still there, since I can expose it by sanding the white/light grey stuff off. Although I'm not looking forward to wet sanding the entire pool surface area by hand, or do I even need to worry about it.

Any ideas?