All I know is that it is a pain! I keep removing it and it grows back very fast.
I agree that its leaf structure is similar to cactus, but it has a shape closer to a certain type of elk antlers
http://www.crmr.com/qeva/Elk.gif
.
Being that it is in a 30 gallon tank I can not put a full sized tang in to see if it will eat it. Someone mentioned to me that there may be a smaller type of purple tang that I may try but I have no idea what the exact name is.
Thank you for replying and I will continue to try for a positive ID.