Blue-ringed octopus（ヒョウモンダコ） in house reef at Bunaken Cha Cha.

Night Dive in front of our resort. When they are calm, you do not see their blue ring straight way, so I thought I found a new looking nudibranch or something, then get closer to them, they start show these beautiful blue rings all over the body and legs.

Blue Ring Octopus　（ヒョウモンダコ）

The blue-ringed octopuses (genus Hapalochlaena) are three (or perhaps four) octopus species that live intide pools and coral reefs in the Pacific and Indian Oceans, from Japan to Australia (mainly around southern New South Wales and South Australia). They are recognized as some of the world's most venomous marine animals. Despite their small size and relatively docile nature, they can prove a danger to humans. They can be recognized by their characteristic blue and black rings and yellowish skin. When the octopus is agitated, the brown patches darken dramatically, and iridescent blue rings or clumps of rings appear and pulsate within the maculae. Typically 50-60 blue rings cover the dorsal and lateral surfaces of the mantle. They hunt small crabs, hermit crabs, and shrimp, and may bite attackers, including humans, if provoked.   (From Wikipedia) 

ヒョウモンダコ（豹紋蛸）は、マダコ亜目 マダコ科 ヒョウモンダコ属に属する4種類のタコの総称。日本ではその中の一種Hapalochlaena fasciata を指す場合が多い。小型だが唾液に猛毒のテトロドトキシンを含むことで知られ、危険なタコとされる。 ウィキペディア