The tremendous capacity of the chromatophore system in cephalopod skin to change its colour across space and time has allowed these animals to produce complex visual displays to conspecific and heterospecific targets, including the aptly-named ‘passing cloud’. This display has been suggested as an interspecies communication, but it has not been investigated in detail. Octopus cyanea produced dark passing clouds during hunting of crab prey, typically after a web-over capture attempt. Clouds moved not randomly but forward along the mantle, over the head and down the outstretched arm web, and were often accompanied by white margins that enhanced their appearance by intensity contrast. This visual display is presumed to be a startle attempt, producing apparent movement by selective sequential chromatophore expansion to induce a crab to move without the drawback of motion by the octopus itself.