Really old thread, I know, but I am having the same problem, and I don't understand why after reading through this whole thread. I'm even finding this slime on the bottom of stems that have not yet formed roots, and it's killing my cuts (in the smaller unit, see below)...they are rotting from the bottom up. This is the stems that are under the neoprene collars being sprayed by the nozzles.
I am actually getting this in my clone boxes, one is an ez clone 128 site, 20 gallon rez (slime on the bottom, sides, and surface of water), the other is a no name brand but similar design, smaller unit:
http://harriganshydroponicsandsupply.com/shop/aeroponics-cloner-systems-accessories/the-clone-king-64-site-aeroponic-cloning-machine/
I don't add any nutrients, just 25 ml of super thrive every rez change
I use AN ph up and down. The only things the labels say is that they contain potassium hydroxide (up), and phosphoric acid (down).
I am adding 5 ml/gal of food grade 35% H2o2 at least twice a week.
I fill the rez with tap water, but ours is really pure.
The things I can think of that could be a factor is that I tend to let the rez go too long between water changes (2-3 weeks), and I changed from AN B52 to super thrive about 6 weeks ago. I also was keeping the ph at 6, since it was my understanding this was ideal for cloning, and perhaps i need better aeration (using a dual outlet whisper 30-60, 4w air pump).
That being said, I would think that with the peroxide being added regularly, the rez should stay clean.
I'll try not adding the super thrive next rez change and see if that makes a difference, but if anyone seeing this has other advice, let me know.
After eliminating the super thrive, I will try going lower on the ph if it doesn't clear it up. I'll post results here if i have any success.