Hmm, I've never noticed anything that looks more substantial, mostly I think a biofilm forms and binds some of the particles together to form clumps but other than that I can't really say I've found anything more interesting than that. I would imagine its a pretty gross place to live, lots of nutrients but not a lot of oxygen. Plus after awhile it becomes pretty stagnant.
Ooh I can't wait to check my skimmer cup when I get home
I think I'm goin on about a month of not emptying it now.......peeeyeew!