Welcome to TFP! East TX, might just be sediment from your water. To test for iron you could rub a Vitamin C tablet on the discolored area. Vitamin C lifts iron.
If you get your water from a well it will be a problem later when you fill. Be sure to update your signature with all of your pool and equipment details. Include your test kit and if you are on a well, list that also.
I think Texas Splash nailed it on the sediment comment. We just "finished" up our pool, but had streaking/staining similar to what I think you're showing. It didn't show up until the first big rain. The plaster should seal all that up.
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