Floating sea grass/algae?

Robust Ghostpipefish - Solenostomus cyanopterus belongs to the family of Seahorses. Unlike Seahorses, it is the female that carries and broods the eggs.

They can be highly variable in colour - green, brown, grey and in combination with white and black and oftentimes, like in this picture, they have moulted look. They swims in a 'swaying' motion like dead leaf, grass or algae as a mimicry.