Agree with hmott - Caulepa prolifera and ditto on it preferring the sand. It's a good well behaved Caulerpa and very efficient nutrient exporter. Very easy to thin it out regularly by pinching the runners in a couple spots. If you exile it to the refugium anyway, make sure you have plenty sand in there for it and keep it thinned out. It needs iron and magnesium supplements down the road, but then, so do corals, so buy the tests and additives for iron and magnesium. Prolifera likes good, bright light, too.