I have been told tht the marks are rust coming through from the concrete frame work , where the reinforcing metal bars are close to the inner edge. it is only a small patch, but a nuisance to remedy. The pool has to be drained to the depth of the area needing attention. Then the tiles removed, treated and then replaced. I am going to get back to the buider to have this done (if I am lucky). But I have heard that it costs around 250 euros to have someone in to do the job otherwise.
I am not having it done this year, so I will keep you posted when I finally get round to sorting this one.