Scientific name: 
Dodecaceria fewkesi
Distinctive features: 
Tiny dark brown plumes extend from white calcareous tubes (often encrusted with pink coralline algae). Similar sp. D. concharum bore into calcareous substrata instead of building tubes.
Size: 
1.6 in. high in large clusters
Dark brown to green or black body. Can form clusters up to 1 m across.