I had feather caulerpa for a short time in my tank when it was cycling. The tank was a 55 gal; the caulerpa came with the live rock. At first the stuff grew like crazy and I was worried. At about 8 weeks I decided to pull most of it out.
I only left a little bit, but the stuff just withered and disappeared. It looked like something was chewing on it, but I never saw it in action. At the time, I had 2 gorilla crabs (I got rid of those destructive critters), a bunch of red porcelain crabs, a percula clown, yellow tail blue damsel, yellow watchman goby, 3 peppermint shrimp, 2 turbo snails (eaten be the gorilla crabs), 5 or six snails that came in on live rock (not sure what they are.). I have a 10 gal refugium full of chaetomorpha.
I have no idea what killed my caulerpa, but I'm glad it's gone.