just seen this thread,and yes i can confirm it is staining from plant-life rotting down over a period of time ,when we marblelight a swimming poo in white l iif its an outdoor pool and the winter cover is a bit late going on you have to make sure no leaves a left as they sink to the floor of the pool and break-down over the winter period and the end results are exactly the same, we also get hair slide's/clips rusting and the rust staining in white pools