I've never grown Caulerpa myself, but a friend with a 90 gallon tank has 150 total watts in lighting over his and it seems to be growing just fine. It is planted in a 4" base of live sand. He harvests overgrown areas by hand about once every two weeks. Since the Caulerpa has begun growing in earnest, he has seen a marked decrease in dissolved nitrate levels. He hasn't done any oxygen level checks, but I'm guessing that they are better than before. Long haired green algae pretty much grows itself, and Turban snails are quite good at mowing it down.
Putting in some live rock is a good start. However, 5 lbs in a 90 gallon tank may not be enough. Also, be sure you are getting properly cured LR if you already have fish in the tank, otherwise you will get an ammonia and nitrite spike as you experiance die back. Also, uncured LR may come with hitchhikers. Some are good, like xenia or copepods. Some are not, like bristleworms that can cause a nasty sting, or mantis shrimp which pack a powerful punch. Some people like Mantis shrimp, though, but that is beside the point. Watch your LR for the next couple of weeks to see if you've got any new critters.
One final thought, a skimmer is often useful in reducing nitrate buildup. There is a downside, though, in that regular water changes are necessary to replace trace minerals that get skimmed. Not a big downside, but I don't know of anyone who looks forward to a water change!
