Well, I agree with the above about regualr water changes, good managmenet, etc - BUT since you asked about what you can use to clean brown algae off of plastic plants - try Jungle Seri-Clean.
Simply tank the plants out of your tank, spray them with Seri-Clean, let it soak for a few minutes, then put them in a bucket, fillit with very hot water, stir the plants around a bit and drain.
Rinse several more times til the water stays clear to be on the safe side.I have used Seri-Clean in the past on some decorations / fake plants from a smaller tank (not my main one now) and it worked wonders - and I never had a problem when the plants went back in the tank affecting the fish