We've had our pool for a few years now (wow time flies)... and my wife has noticed that the horizontal surfaces are starting to develop darker spots in the pebbletec. She is going to call the guy who did the install but I'm curious of other ideas. To me it looks like the light colored plaster that binds up the stones is wearing away ever so slightly and more dark stone is showing now. It is also possible that the light colored part is staining somehow. I thought that the fact that this is more noticeable on the horizontal surfaces could mean something is settling out and staining.
I think our chemistry has been OK. We have had several bouts with heavy rain, pumping and needing to work to get the pool to hold chlorine levels again, but it's never turned green and we have always gotten it to balance out again in a few days. I'm not sure what else to check. Hope the installer has ideas. It is not bad now but I'd like to stop whatever is happening from getting worse.