Hi. I'm just a first time owner myself, so I probably can't give you much helpful info. But from personal experience, Milky can go from playful to agressive very quickly if he doesn't like what we're doing. By agressive I mean he will suddenly bite our hands or legs or whatever it is he's not liking at that moment. I don't think it's sulky, but if he doesn't like something, he's not afraid to show it. It can change in the blink of an eye.
Maybe you should try to watch and understand his expressions so you can avoid Ashley getting "moody/sulky". See when and how it occurs, what expressions does he make, how do his ears/eyes change etc? By determining these, perhaps it will be easier to find out the reason behind why he does it. I know it's not much info, but hope it helps in some way.