Are you sure the white powder is DE? I have had a similar problem with a white slimy powder accumulating on the bottom of my spa and the pool around the returns. I also thought it was DE and spent a lot of time with the filter trying to figure out if it had a leak or something with no luck.
I carefully scooped some into a jar underwater, then poured it through a coffee filter to collect it. The powder does not feel gritty like DE, it is slimy. I was wondering if it could be dead algae.
I just turned my Solar heating off for the winter about 3 weeks ago and the white powder is not returning. Could it be something caused by heat or a chemical reaction caused by heat and/or the Solar panels?