ding* ding* ding* food and junk is the correct answer. i got tired of wondering what it was and decided to clean out the tubes today. this pic is after one swipe into about the first 6 inches of tube! i ended up tying a string to the pipe cleaner thing from my emperor and made passes through the tube to clean it all out. yuck. interestingly, the gunk in the tube back into the tank was considerably lighter in color than out of the intake tube...
i'm going to chalk this up to over-feading and re-adjust my feeding schedule. there was also a lot of sand, so i shortened the blue intake piece by one link.
30 Gallon FW. 1 Albino Bushynose Plecostomus, 2 Harlequin Raspboras, and 1 Male German Blue Ram