White Build up on pool tile

I have been battling a white build up just above the water line on our tiles. I aso noticed today in one area a solid white hard substance forming in some of the tile grout.

We have been battling for about two weeks now high CYA and low FC, but finally are getting that under control.

Today
CYA - 60
FC - 5ppm
TC - 5ppm
pH - 7.6
Hardness 150

The pool is only 4 months old, so I was told by my builder not to worry about hardness levels being low until the 6 month mark and to just keep brushing the pool every day (do you agree????).

I bought today a pumice stone "Original Pool Blok" to remove the build up, while it worked, I am going to need a whole lot more to finish the job.

I was wondering what has caused this build up (does it have anything to do with the previously 100+ CYA level) so I can avoid this very labor intensive process of removal in the future!

I have pictures if that will help...