Sounds like dust stirred up by lawnmowing, happens at our house all the time.
When you cut your lawn, old, dead cuttings pretty much pulverize into a fine dust, this dust will immediately cling to the surface and eventually sink to the bottom, currents in the pool will naturally fill in crevices with it.
The problem with the fine dust is that it will push trhough pretty much any sand filter unless you reduce the flow to almost nothing.
Try 1-1.5 cups DE right before vacuuming, vacuum VERY SLOWLY, too much speed and all you do is stir the stuff up, and mix it back into the surrounding water.
I don't know about where you are located, but where we are it has been EXTREMELY dry which certainly isn't helping the dust situation any.