I know this is an old post, but I am having a similar issue with same pebble plaster pictured in the original post except mine is already 12 years old. This year, I had the pool acid washed. The acid was diluted propery from what I know. The pitting isn't really terrible, a lot of pebbles come loose apparently when you acid wash, but the discoloration started to appear mainly in two small areas over the course of the season. I have never had a plaster issue before. My pool guy insists that it's not the acid wash - and maybe he's 100% correct - but I have no other explanation. Yet. I bought an underwater camera but haven't yet developed the film. In the meantime, can any of you here shed light on acid washing pebble plaster. I've read a lot of posts where pool experts warn against it. My pool guy assures me that it's a lot of chatter, and, that done right, acid washing pebble is fine. I treasure this guy, he's hard working and honest. It's not a good spot to be in. I just want to figure out what is going on.

Does anyone know if this is a concern? Thanks.

It looks like pieces of white plaster in the mix. It can be from the the mixer, trowel, or something along those lines. After repaired those areas should not be visible. Highly unlikely it is from the manufacturer as the aggregate is screened and the "impurity" is much larger than the rest of the aggregate. The pebble finish was applied in the day time, if the unevenness in not visible in the day time when the pool is full of water not much can be done. The pool light can cause very small differences to be exaggerated due to shadowing. Sometimes adjusting the light can help. Hope this helped.

Did you ask your pool builder of the folks at NPT? We had some brown spots in our pebble finish that we were worried about and our pool builder came over to check it out for us.