Who is supposed to be maintaining the pool? If pool builder, than follow his instruction for warranty. If it was my pool, I would just add some bleach/chlorine. Have you bought a Pool Test Kit, yet?
Those rocks look like Moss Rocks. I also have Moss Rocks for my waterfall and part of my pool wall. Are you sure it is algae and not moss starting to grow. I am pretty sure you have nothing growing below the waterline. I use a spray bottle full of bleach and a nylon brush to remove the moss off...
A 25% increased on your normal filter psi. For example: At 8 psi, you would backwash at approximately 10 psi. A 2 psi increased of 8 psi is approximately 25%. I am an old fart, so I am using math from the 60's and 70's. Good luck
I am a noobe, but after cleaning my Pentair DE Plus Filter 2 days ago, the spring with the brass nut (picture #2) should be compress together. I will let the experts chime in and correct me if I am wrong.
I have 16' x 34' freeform(kidney) pool that is 4' - 6" deep. I should have gone deeper, due to living in the Houston, Texas area. Water warms up in the summer.