Most likely Staurocladia oahuensis, a species specific to the Pacific.
Hydrozoan
Details
Scientific Name: Staurocladia oahuensis
Identification: Looks like a small starfish with small sticky tentacles. They appear during the early stages of a tank that has new live rock.
Pest Management
Treatment: They are harmless and will disappear after a few weeks or when medium sized fish are introduced.
Prevention: