Rabbitfish in reefs can be risky. Some may nibble on LPS, while others leave them alone. That being said, I have had a Doliatus Rabbitfish in my reef tank for over 5 years and it has been fine. If you are talking about grape caulerpa, yes, it will mow it down. If you are speaking of Valonia, mine will eat the smaller ones, but I have to remove the larger ones by hand. Keep in mind that these fish can get pretty big and can also produce a lot of waste.