It's grape caulerpa. It does grow very and you either need to get fish to eat it or watch out that it doesn't go asexual and start to decay. You should prune it every so often.
be careful,I had this in my old 58gallon reef and without pruning it will take a tank over.the roots run deep and wrap themselves around the rocks,it could be a real chore to get it out once it gets to that point.