Sargassum
Details
Scientific Name:
Identification: This is probably a species of brown algae known as Sargassum. It is indeed a brown alga, which if given the chance can grow quite well, possibly shading out other light loving organisms. Otherwise it is unobtrusive, and wont damage large polyped corals as Caulerpa might.
Pest Management
Treatment: It may be eaten by some tangs, but it may be more accurate that some individual tangs or rabbitfish may eat it. You are more likely going to have to prune it by hand.
Prevention: