halmidia will take up some nutrients, but it's mainly for looks. one of the few things you can grow in a reef tank that looks like a "plant". It is way easier to control than caulerpa. downside is that will take up calcium, but when it dies it leaves you with aragonite.
here is a link with more than you want to know if interested.
http://www.reefkeeping.com/issues/20...nftt/index.php
#4 is yours.