No expert here, but I would consider that a defect in material and/or workmanship (probably both). And I would expect that to be redone under warranty. And I would demand a different, better crew perform the work.
Plaster does mottle over time. Mine is already, and it's just over a year old. So you have to expect and accept that. But you have to really look for it to see it, and it mostly shows up under certain lighting conditions. That's not what's going on in your pool.
A word of caution: if he's like most PBs, he'll first try to convince you to let him acid wash your pool. And acid washing is a legitimate part of new installations of aggregate-type pool surface (like pebble). But if it's plaster, or pebble that has already had its initial acid wash, then I wouldn't permit this solution. Not without a very significant reduction is price. Acid washing works by removing plaster and taking offending stains with it. If you allow an acid wash, you're going to end up with less plaster than you started with, paid for. Great quickie solution for the PB/plasterer, because they can drive away and forget about your pool. But years from now, when your finish is aging and it's no longer under warranty, you're going to miss that missing plaster! Why should you pay later, with years, or even months, of longevity, for their mistake.
