Caulerpa macroalgae. It looks like the racemosa species. It is also known as grape caulerpa. Make sure you trim it frequently and if it ever starts looking translucent, remove it from the tank immediately.
This plant has a lot of info about it and thus there are plenty of myths about it. Just do some research and you will do fine. I have 2 subspecies of this plant in my tank and love them.
ps- I wrote a detailed post on this plant in this forum, if you feel like searching through 800+ posts of mine, I'm sure it will help you.