'sea grapes' = Caulerpa racemosa
You'll find alot more info with the scientific name.
I have some in my tank and it grows very nicely. also looks good! Almost everyone here will tell you that you shouldn't keep them because they, as all caulerpas can, will go sexual. It has never been my experience, thankfully.
Do a search for caulerpa racemosa and you will get all the info you would ever want!