Hey all. Former professional now doing some independent maintenance for some family friends. However, my experience is mostly with gunite/hydrazzo/pebbletech/ANYTHING other than vinyl liners and fiberglass pools, for the most part.
Anyway, these friends have a liner pool, and they have developed some sort of stain, I believe. The homeowner is under the impression it is 'mold' but...I doubt it. Anyone have any idea what this could be? When I nylon-brushed it, it just laughed at me. It is a brown-grey color. At first I thought metals, but as you can see the stairs (as well as the returns/UWF) are bright white, so that seems unlikely. What options does this leave? Organic stain, perhaps?
Also, I do not have a chemical reading yet as they only have test strips (which look like a bit of water got in the tube anyway). I am ordering a Taylor K-2005 and I will post the results here in a week or so once it arrives in the mail. But I am nearly positive that the chemicals are severely low, as I noticed they haven't bought any chemicals since the last time I came over and shocked their pool two months ago, and that shock was the only chemical they had at that point anyway. So I'm sure the water is extremely aggressive and will need tons of TLC.
But anyway, here are the pics. The 2nd is a zoomed-in shot of the 1st, and the 4th zoomed-in of the third. Thanks for all the help to come!
Next two in comment.