If you used L/Rock in the system or added a coral on plug you may have introduced it to your system. Noting eats it period its rumored a type of slug will ? None are proven to though. It will kill anything it touches I have a fire coral that totally grew around a littler tuft of it..
I'm debating now to just rip my reef down and cut my losses.. All I had a was a little speck on a coral I thats I transfered to the new one.. But it was on L/Rock that was sued 7 yrs ago in a 90 she had set up.. There are people who will say that magnesium Chloride and or Kent's Tech M will kill it ? I tried both nothing P.H at 8.9 9.2 for a few weeks nothing it would weaken and turn Brown and all but not die. Its a rich dark green with a feathery looking leaf, Have a peek
Caulerpa taxifolia