No idea why this thing is called the Yellow Boxfish! But these bright yellow, polka-dotted, boxy balloon-shaped fish are found in tropical and temperate marine waters of the Indo-West Pacific.---maybe because it's yellow and box like?
Nudibranch by Dermal Denticles via ferrebeekeeper: Gastropod mollusks - Range: shallow intertidal surf to Most are benthic seafloor dwellers, but some float upside down at ocean's surface or swim in water column.
This animal, the Leafy Seadragon, is a Threatened species. They are threatened by collecting for use in Chinese medicine (and by habitat loss). Just one more example of the beautiful natural world exploited by idiotic traditional Chinese cruelty.