if you have caulerpa in a tank under 75 gallons, the ONLY way to rid yourself of it is simple: take rock by rock and pull it out as much as you can, and also get it out of your sump. Run a GFO (Phosban) reactor. Every week at first, you'll have to disassemble the rockwork and pull it out. Eventually you get down to once in a while. But probably with those rocks you will occasionally have to pull out a strand of caulerpa until the heat death of the universe. How do I know? I have the stuff, which is an absolute plague.
the downsides of caulerpa: toxic to most fish and inverts. The only fish that do eat it are inappropriate for under 100 gallons and will kill your smaller fish. And if subjected to stressful conditions, it will spore, turning your water cloudy and killing everything in the tank. It reproduces by: fragmentation, spores, runners, roots, and is illegal to discard in California.