Help,not sure what this is on bottom of pool

[matt4x4]

Waste - Thanks for that, noe I gotta go get some paper towels and clean the coffee off my monitor!!!! LMAO
With regards to your question, I'm in southern Ontario (canada), Now having reread the posts from tuesday on, I'm beginning to think it does have something to do with people's locations being closer to rural than urban, we have lots of "feed corn" fields around our area, the soil is high in clay content, however, the "mud" is definitely not inert clay dust and could never multiply that much.
Around here, farmers also fertilize in spring, but it's usually so early, your pool is likely not open then, farmers around here also fertilize with liquid manure during fall before they turn the fields over - this was the case a few weeks ago, since it really stunk for a few days.

From my experience, this is what I have leaned in 3 years about the "mud":
Year one - nothing, thought it was dirt, then it was too late - shock shock shock, brush brush brush...
Year two - remembered year one, shock shock shock, brush brush brush but only to the typical 15 ppm, nothing really changed other than the mud very slowly became more mud - slower than year one. Finally had enough, it was end of season anyways, I dumped in my remaining chlorine and voila, within a few days, the mud was killed off.
Year three - this year, almost like clockwork, same time, different year, I started seeing our old friend "mud" again, this time, being wise about how mr mud works, I immediately dumped everything I had in the pool, brushed vigorously for about 2 days and all was well.

Swimmer load does have something to do with it too, I think this stuff set in when the circulation of the water is less optimal, we stopped using the pool on a regualr basis after the first week in august, meaning the water does circulate less, however, I was on top of my chlorine levels DAILY even though we were using the pool less.
First signs of this mud are always past the far side of the steps from the return (my worst spot for good circulation).

At this point, I'm just glad i have a way to control it, 20-25 PPM does seem to be kind of high, but heck, if 15ppm doesn't do it and 25ppm does, it's an easy fix and I'm not questioning it.
Since my chlorine levels and all other levels were in perfect range this year for sure, I can also determine that this stuff is pretty resilient to the effects of normal chlorine levels.

Oh, I wrote some more here:
http://www.poolforum.com/pf2/showthread.php?t=5612