I'm on the Colonix and have been getting some mucoid plaque. It's definitely not the fiber in the drink. I can clearly see the difference as you stated in your post. It is rubbery and distinctly darker than the gelled stuff. (stinky too)
I was curious about something you posted. You mentioned clay in Colonix. I checked and double checked the ingredients on the Colonix and there is no mention of it containing clay at all.
There is mention of guar gum as an ingredient on the label, which is a thickening agent, but some research on that appears to indicate that it is something that will gel similar to the psyllium that is in Colonix.
The main reason I chose Colonix over the Bentonite clay mixtures people are taking is that I didn't want to be fooled into thinking I was getting out mucoid plaque when maybe it wasn't.