I'm not sure where to post my question. I have a chlorine pool following the TFP method since August 2015 and everything has been great.
I've notice on my travertine spillway in the corner a substance that started off mostly white and now is looking white/blue. It is hard since I tried to brush it and nothing happened. It almost looks like it is coming from the area between the travertine piece and the side piece of travertine, if that makes sense. See attached picture. Any ideas on what it is or how to correct it?