Our pool and spa are less than 1 year old. The plaster is cody pebble and I recently noticed that the bottom 6 inches of the side wall in the spa is yellowish. We had an issue with an electronic valve that drained the spa briefly last week so I poured bleach along the yellow ring prior to re-filling the spa but that didn't seem to diminish the yellowing. The valve problem did diminish chlorine levels briefly but that was after I had noticed the yellowing. Any suggestions? Many Thanks from Austin