Tidepool Tim says,  “Coral crust or red crust does not look like a type of algae or seaweed, but it is!  It is hard and grows as a thin layer over rocks, shells, and other bottom structures.  Some bottom dwelling fish such as sculpins will have this growing on their skin. Chitons, crabs, and shrimp sometimes have a layer of this red coral covering parts of their bodies.  Coral crust grows in 2 forms; first a crust as described above but also as a fruiting cluster that resembles broccoli florets. We call these coral clusters and even "live rock" - these clusters are interesting as they form a micro-habitat for tiny sea urchis, scaleworms, limpets, crabs, etc.  These can be ordered for use in ecology labs and experiments.”