I was told once that high total alkalinity can lead to this, along with very hard water. I am surprised that the acid didn't remove it though... that works great on mine.

Are your tiles glazed? There are some good tile cleaners available, but you need to be careful with tile that is not glazed ... the cleaners can etch the surface (which ironically doesn't happen with the acid ... I found out the hard way).

Have your water tested. High TDS(total dissolved solids), high PH(acid reading of more than 7.6), high calcium hardness (CH of over 500ppm generally). A combination of these can lead to deposits on the tile and/or plaster.
Also have your tap (make up) water checked to see what the TDS and CH is. High levels in the tap water means you should also be using a product to keep the solids/calcium in suspension.
TA(total alkalinity) should be between 80-120 ppm. TA is kept in check with muriatic acid. As the ph is lowered to the 7-7.4 range then the TA will start to be lowered.
Tile cleaning by professional using glass beads or equivalent is the best method to remove build-up on the tile. Good luck.

Here in Phoenix, pretty much everyone I know besides me has that line. Not only did I take my pool off autofill so the water never sits in the same spot constantly, I added two bags of salt to my traditional chlorine pool. May not be proper, but my pool looks great!