I was dealing with a lengthy illness and wasn't caring for my tank besides the minimums, and this plant has overgrown everywhere in my salt water tank. It's a very green plant (not sure why the picture looks so yellow).
It is a caulerpa algae, although it has roots, stems and leaves ( may be prolifera or floridiana ). Tangs, Moorish Idols and other fish eat it. It grew very vigorously in my aquariums and so can shade or choke out corals IF not trimmed. In a 75 and a 55 I grew enough to supply a couple independent pet shops. As you note, it should look very bright green.