ok...jumped in 60 degree water this afternoon with goggles and eyedropper to get some samples.
the reddish-brownish patches on the pool bottom mostly appeared to vaporize when i disturbed them, but using the eyedropper, i was able to "suck up" some samples...it almost seemed fibrous or "pulpy like".
once i colleted some of this in a jar, i let it settle and then poured the sample through a white paper towel.
i have to say it sure doesn't look like sand....seems more like a gummy dirt when concentrated together.
could this be pollens, seeds, etc. brought in by the wind to settle on the bottom?
i still don't know...
but, i'm pretty sure it's not sand or mustard algae.