Well, no good deed goes unpunished.
Yesterday I have bought some lawn fertilizer and applied around the lawn including next to the pll deck.
When I was swimming late in the evening I have noticed some brownish areas on the pebble walls. Initially I thought it was just dirt and I need to brush and vacuum. To my horror I realized that I can't just brush off those spots and I thought that it may be mustard algae. I dumped four bottles of bleach and was preparing to start the algae battle in the morning when I noticed round brown spots on the deck.
It was the lawn fertilizer that was causing these pool and deck stains!
When I bought it , i saw that it has high iron content, but I could not imagine the it will cause such a heavy staining such quickly.
So, lesson for the future - be careful what you spread around the deck and the pool.