George,
I would first try an in water acid bath.
Turn off the pool equipment and set it so it will not come on.
Add four gallons of acid for every 10,000 gallons
Leave the acid in the water for 4 days
Brush the plaster with a stainless steel brush at least once a day
After two days check to see if the deposits are loosening you might have to add 2 more gallons of acid for every 10,000 gallons
When completed, bring the water back to a neutral level adjust the ph level and the alkalinity
Start up the pool
These steps should bring the pool b ack to its original color. I do this on every quartz plaster I install after the the plaster has cured for 30 days. It is not uncommon to have to do this to quartz plaster every now and then as it is prone to calcium and other deposit build up.
Troy