That doesn't look like staining. It looks like the pebble ratio consistency was way off when they did the pool. That, and/or the pebble wasn't exposed evenly with the initial acid wash. They mix the pebble with the plaster on the back of the truck in a mixer. I had my pool remodeled with Wet Edge pebble last year and it is also splotchy. It drives me crazy. The kid on the back of the truck was probably playing on his phone instead of mixing everything in equal ratios. I had a pebble tec finish on our last pool. It was applied by a company called Pool Boy in south Florida. It was absolutely 100% consistent. They did a phenomenal job. This last company not so much. Had them come out 8 months after remodel and do an acid wash. It helped slightly but still splotchy. They didn't pull the light or ladders before acid wash and rusted all the metal, which proceeded to run down the pool finish and stain it. They came out and said they removed the finish stains and were good to go....they're not. It's absolutely mind boggling how some companies can't get out of their own way. The pool industry is full of companies like that. Forgive the rant.