Brown gunk in tank! Please help!

Well, you have to remember that when a lot of plecos get older, they do not like to eat algae as much as they did when they was young, hence the need for feeding fresh vegetables. You can either get a pleco like a bristlenose pleco who will pretty much always eat the algae, some otos who do wonders in eating algae, or you can just clean everything off every week.

Here is some algae info that is pretty good. I used to get this brown gunk that would grow on rocks, glass, and decorations. I think it was because I fed my fish too much and didn't do enough water changes. My new tank so far has been crystal clear with no brown junk. From the reading I think it was the brown algae however it wasn't on a fresh tank and definetly didn't dissapear by itself.

http://www.aquaticscape.com/information/algae/algae.htm
http://www.dupla.com/e025.htm
http://www.vectrapoint.com/main/manual/bm36.html
http://www.drhelm.com/aquarium/algae.html

Not a disease at all. Just an algae growth. As suggested you could simply clean your decorations every so often. If this has not been a problem before, then may be experiencing elevated nutrient levels. Obviously take car of the usual like cleaning the filters and gravel, making sure not to overfeed, and doing water changes. However, another possible cause that often gets overlooked in older tanks is the size of your fish. Depending on what species you keep and how big they have gotten, the larger sizes and subsequent apetites could be responsible for added organics that are serving as "algae food."