Helmet urchins feed on algae at the water’s edge where they move about on little tube legs. I can’t claim to have witnessed any such activity, but for creatures that apparently just sit there, I find them quite cheerful and entertaining. Perhaps I should get out more.
To identify this, I used John P. Hoover’s book Hawai‘i’s Sea Creatures: A Guide to Hawai‘i’s Marine Invertebrates. His website is hawaiisfishes.com.