Back when I first got started, I had the same stuff growing on my new live rock. Can't remember where I found out about it, but I was under the impression that it was sargassum, but I could be wrong. Either way, at times it can grow pretty quickly so just prune it when needed. If you don't want it at all, just try to pull it out of the rock from its base and throw it in your trash (I kept doing this and eventually it was all gone). If you don't mind it in the tank, I'd just prune it as it helps in nutrient export.