The ID is tentative. These anemones are not especially uncommon, usually attached to leafy algae in lagoon Halimeda patches. Most often during the day only the base shows but the tentacles will come out at night. The bases are usually around 20mm or less.
These with transparent tentacles may be smaller specimens.
The one below was identified on iNaturalist as a young Bunodeopsis medusoides.
Created 25 July 2018
Updated 24 May 2023