I had a heckuva time trying to figure out what this coral was called. It was in one of my general reference photos, and I thought it was so unique I had to include it in my drawing. Well, I needed more than the one tiny photo, so I had to figure out what it was. After pouring through dozens of photos in several databases, I discovered it is a red soft coral, genus Dendronephthya. There are so many species in this genus nobody would list them all (that I could find).
Like hard corals, soft coral is an animal, not a plant. But unlike hard coral, they consist of a group of polyps (the red parts of this one, which are individual invertibrates) and the sclerite, or crystallized calcite, at their base that holds them together. The white part is a flexible "inorganic matrix" of material to support them. Hard corals also have polyps, but they and their structural support are, well, hard. Soft coral primarily feed on algae and plankton.
This coral was actually pretty simple to draw. Once I had the shape, I added the red polyps with crimson red, and where they were in shadow I used tuscan red layered with crimson. They are just tiny dots, really. The white stalk is, of course, not white. Shadow is achieved with slate grey burnished with white. It results in a very slight blue hue.