The first few photos closely match published figures of Ophiarthrum elegans (now moved to Ophiomastix). Most of these have been found under rocks on Kwajalein Atoll lagoon reefs. The first photo below by Stan Jazwinski appears to be the adult form.
The rest have some differences in disk and arm coloration, but we are assuming they are variations of Ophiomastix elegans.
Created 16 January 2019
Updated 26 June 2021