Grape caulerpa, transfer it to your sump. As Flower said it grows very fast and once it gets a foothold in your tank it can be near impossible to remove (even with tangs as a few of my friends discovered).
I used to have this growing in my overflow box and it would span out across the water's surface. I clipped it back pretty often since it grows fast. The fish stores were thankful to get it!
Grape Calurpa.
Where is your live rock from? Around FL?
