Ohhhhhhhh, you're supposed to clean them...
The brown stuff is probably some sort of algae. My fake plants have gotten really dirty, but of course I wouldn't think to clean them. : Do you have to though? I actually think they look better, more natural, with algae on them.
LOL, the brownish patches on your fake plant are algae I know this because in the past when I had only fake plants, and no live ones, I had the same problem. You should clean them every once in a while - that's all. The reason algae often start growing on fake plants is that they're not a very accessible place for an algae eater to clean them. It's much easier for an algae eater to eat algae off the glass rather thank off a fake plant. So, as a result, you end up having algae growing on fake plants but not on the glass or anywhere else in a tank.
yep they get algae really bad. This is what I do, swish them in plain water to get any debris off of them. Then soak them in a bleach solution for about an hour. Then rinse in tap water to get most of the bleach solution off and then soak in a dish full of heavily dechlorinated water for about an hour. if they still smell like bleach when you take them out soak a little longer in dechlorinated water.
This will kill any beneficial bacteria growing on the artificial plants. I only have a couple of them, but if you have a whole tank full , don't do the whole tankfull at one time.