It is definitely mineral deposits. It was fine the first 2 years of pool ownership. This year we had only one rainstorm in the summer. FYI I am in Phoenix. We had over 32 days of 110 degree weather. I don't know if that makes a difference or not.
Don't know if it's the best way, but when I clean mineral deposits, I use a quart-size trigger sprayer bottle. I use a 4 parts water, 1 part muriatic acid mixture. Some people start with more water (less dilute mixture). I make up about a cup of the mixture because that's usually more than enough for the few areas I need to treat. Always add acid to the water in the sprayer, not the reverse. I don rubber gloves and eye protection and hop in the pool. (What a sight, I know!! )
You can wait until you're in the pool to add the acid to the water in the sprayer...that way there's not really any concern if you spill a bit...it just lands in the pool.
Then, I spray a small area, let it bubble on the surface, scrub with a hand brush. Repeat as necessary. Rinse the area with pool water when you're done. When everything's done, I empty any leftover mixture into the pool and use pool water to rinse out my brush and sprayer, including the trigger mechanism. Then I take a nice relaxing swim as my reward :-D