The yellow stuff could very well be pollen at this time of year depending on where you live. Looks like a fine dust that is stirred up easily and will go right through a sand filter. Dug up this pic from last May when I had a really bad case of the stuff. Sucked some off the bottom of the pool near a seam where it settled, then looked at it under a microscope. Pretty obvious what it was. The lower right panel shows the calibrations for the other fields.
Al

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