That is Palythoa caesia. Some call them Pillow Zoanthids, but they are actually a Palythoa strain.
I have a few cool morphs in the past. The coolest was a green oral disc when the polyps opened while the rest was the tan color. They can be fast to slow growers. I currently have a pretty slow grower in my tank now. They are pretty hardy and my mother colony has been with me for close to 3 years.
Kevin
http://www.marinelifephotography.com/marine/cnidaria/palythoa-caesia.htm