Is this algae? Or problem with DE filter?

[Vinyl_Guy]

My thoughts exactly Mom.

Denanbob, are there actual physical depressions where this "gunk' is collecting? They don't have to be very obvious or pronounced just slightly varied.

You've got something going on with the green build up, which might be some how making it back into the pool. I'm not a chemical guy, I'm more of a nuts and bolts one, so I can't help you on what the DE is extracting from the pool. But, have you checked for a small tear in the DE grids, or some of the stitching letting loose? Is your mulitport, or whatever valve you use seated properly after you back wash? It could be sucking residue from you backwash line. Is the back wash line clear and free and not "backing up"?

Added: Have you rinsed the bottom of your DE filter body, out the winterizing drain?

For the short term it appears the volume of "gunk" is so small that a quick vacuum of the pool before company would suffice. Even a quick brushing should cause it to suspend for a while. If there's any activity in the pool in that area I doubt you'll see it most of the day. Hope for sun and hot day, with 21 people I doubt it will have a chance to settle. ;-)