Backwash starts out cloudy but didn't take long to clear, then I remember Ben saying since the DE just coats the top of the sand it wouldn't take much to backwash it. That clumps of goo that I assume is the dead algae is pretty clear, if its not in the water it's hard to see. I do still have some stuff that probably is passing thru the DE but did you have that as well and the poly pretty much polished your water?