At least some of these appear to be the species known as the algae octopus in Indonesia and the Philippines. At Kwajalein they are small and live in Halimeda algae patches, where they are hard to find. When caught out on the sand, they will curl and hold up their two anterior arms, looking like they are getting ready to box. I suspect there may be multiple species among the photos below, but so far have not noticed any reliable way to differentiate them.
Created 13 September 2015
Updated 26 June 2021