This is not a rock, this is not algae, this is a colonial animal known as coral. This particular specimen is one of only 3 colonies on a secret patch reef in Hawaii. Scientist called it Montipora diltata or the purple rice coral. Environmentalist, scientist and other concerned specialists have argued over the proper identification of the coral (even the genetics are contentious), which is preventing from gaining endangered species status (and the protection that entails). As of right now it is on NOAAs species of concern list.
I felt very lucky to get to see this species while it was still around. Its bright fluorescent purple color really help it stand out from the rest of the coral (we could even see it from the boat).