I noticed this in the tanks a few days ago, and thought it would go away on its own but it hasn't. I have done a 100% water chang in one tank, and the other I am leaving till tomorrow (fishies are in their cups in clean water) when I can take some water samples to the pet store. Its gross, and a brownish color. It is on the walls of the tank, the terra cotta pot, the rocks and its almost eating away at the plants we have in there.
Pictures, I only have 2