Hi, Tom,

I wish I could offer some practical advice for getting whatever it is off the wall.....I just have no experience. Home Depot sells "sulfamic acid" a very mild acid for cleaning masonry.......I just don't know if it would have an effect on your liner if you brushed a little on. Smarter folks than me may have an opinion.

Yeah, your pH and Alk numbers are up there pretty high.....very conducive to scale formation. Adding some muriatic acid would help a lot

I believe I would work on reducing pH to around 7.2 - 7.6 and Alk down to the 90 -120 range........that would probably prevent you from ever geting back into that scaling condition.

I have asked several pool stores about adding DE to the sand but they never heard of it. Though they have heard that their $23 Sand Filter Cleaner works well ;-)
Sounds like your water is looking pretty good....nice work. I do believe you need to add some muriatic, however.