These look like figures of both Ctenactis echinata and Ctenactis crassa, but appear to have a single mouth in the middorsal dorsal groove, a character of C. echinata. The photos below are all from various Kwajalein Atoll lagoon pinnacles.
This appears to be the same, although the central groove does not look as though it extends as close to the ends as our other photos. The tentacles were extended on this night shot.
We are guessing that this could be a young juvenile but the ID remains tentative.
Created 10 April 2020
Updated 9 June 2023