Richard, My guess is it's algae, it multiplies if left alone to the point where you get the heavy deposits in the craters getting even heavier, and an even film over the rest of the floor, I guess what i mean by "telltale blue" is the cloudy/white, but mixed with the background of the blue liner, it looks blue to me.
I will recheck my CYA tonite since i just dealt with this crap last week and it won't have changed from tehn, but I'm guessing it's somewhere closer to 20 than 30.

Even with DE in my filter, some (a lot) will get filtered out and looks green upon backwash, however, it comes back just as quick, and like i stated before 15PPM only slows it down, 20-25 has an effect. Dirt won't oxydize, so i know it's not clay dust - nor would my pool collect that much of it so quickly with a cover on it. And backwashing would be brown, not green.

Now, here's another observation my neighbour and i had the other evening - the air was just THICK with microscopic flying bugs, and I mean thick! No, it was not the beer we were drinking!
We couldn't see them if having the sky as the backdrop, however, get the trees as backdrop and you could see these things EVERYWHERE - now, maybe they're not the cause, but I could see that they may be the food keeping this stuff going.

Also, this year, I added salt to my pool, I thought salt was supposed to help deter algae as well, obviously not this stuff....