Seen during a black water dive, this is most likely a juvenile of Bentstick Pipefish during pelagic phase. Despite a juvenile, it is around 12-15 cm in length but very slender/thin. It has 2 long appendages behind its head and a few others towards the tail. The adult of this species looks more 'ordinary' and usually found on sandy and seagrass bottom, looking like a Stick, hence its common name, Bentstick.
During juvenile phase, they floats in the water column and are active during night time feeding on planktons and other tiny critters that were attracted by the lights we put out during the black water dives.