Assuming that it is from the Carribean, it is either Neogonodactylus wennerae or N. bredini. Given that N. bredini rarely show up in live rock because they only occure shallow, it is probably N. wennerae. It will live to a maximum age of 5-6 years and reach about 6-7 cm total length. This species will frequently change color in an aquarium, so don't be surprised if it turns a lighter green.
Roy