Lyngbya majuscula
Description: filamentous seaweed black to dark grey but can be shades or red green or yellow. 5-10cm long may cause swimmers itch, found with other seaweeds.
Habitat: shallow
tide pools and subtidal habitats.
(In: Hawai Coral Reef Network - http://www.coralreefs.hawaii.edu/ReefNetwork/marlife/seaweeds/seaweeds%20descriptions.htm)