There are a few different styles of Birdsnest coral (Seriatopora), and this one is a crowd pleaser. That bright green really stands out in the reef, and while the skeleton within is quite thin, the polyps add some heft to make it appear larger than it is. Birdsnest is rather brittle, easy to frag, and a great choice for someone wanting to try out reef keeping.
In the above picture, there are actually three different types of birdsnest, all equally beautiful.
Photo by Duane Oestreich