I just replied to the other thread. Copying here with some details for context.
My builder skipped town right as my pool was finishing and I didn't know enough about chemistry to balance my water properly. So I had staining. Lots of white patches on blue plaster. After a year, I started researching what to do about it.
I ended up doing a no-drain acid wash with Jack's Magic Copper & Scale Stuff. It made a HUGE difference. Make sure you bypass your heater or it will eat right through the copper line. And make sure you leave the pump running continuously while doing the treatment - movement is very important for Jack's to work. I ended it after about 5 days and my plaster looks fantastic. Some of the staining has come back, but only in spots where I have something sitting against the plaster (ie, chairs in the beach area).
One thing to note - the stains on your plaster are likely very high in calcium, so your hardness it going to go through the roof. I ended up with a hardness around 800 and had to drain my pool to bring it down.