I suggest that you play around with not just making each shade darker, but also play around with shift the hue values. Not much, just a tad to blue as it get darker. It adds a really nice touch. Experiment with that and you might like it.
Besides that, I feel that the dithering adds a wrong feeling for an octopus. Not that I know much about octopi, but they always seemed like they would be slimy, if they are cartoony, not grainy, with dithering gives the feeling of. Or, maybe you should look into how some real octopi are textured.
Also, the head doesn't seem round enough. Like it flattens out too much where his face is.