Amphioctopus marginatus a.k.a. coconut octopus or veined octopus. I’ve seen them a lot in the Philippines and Indonesia.
Their color and shape abilities are amazing, but they also have weird habits: They usually build a home out of shells, coconuts and debris - and once they feel disturbed, they would just take their stuff with them and walk along the sea floor. Funny scene.
I tried to get the best lighting possible for each shot - with very mixed results. Here's a test with ambient light vs. video light (SeaLife Sea Dragon 2000):
Also check out the video, mostly filmed with video lights: