razor caulerpa. Can be tough to eradicate. Lots of herbivores won't touch it. It's much tougher in texture than other caulerpa varieties. As suggested, manually plucking it may be your best bet. You should at least be able to keep it under control. One thing every reefer has in their "toolbox" is a good pair of 6 inch stainless tweezers. It's a very tedious process, but you just have to stick with it. Just remember that any piece of the rhizome that still has roots attached to it that you leave behind will just keep sprouting and running. So start at the very beginning of a runner and gently pull back without snapping it. It takes a bit of patience.