If it IS DE, flocculant will NOT help. DE settles more quickly than flocc-ed algae . . . if you turn the pump off.

I haven't read this thread carefully, but I read enough to know that you need to verify that it IS DE, and not dead algae. If you grab a pinch, DE will have texture and substance; dead algae will not.

And IF it IS DE, something's wrong with the filter, though it could just be a mis-oriented valve.