(disclaimer; I own none of these images)
Little Shop of Horrors - A zoanthid that resembles the plant from the musical. My favorite zoanthid... if I could actually keep them alive. I'm still trying to solve why I fail at keeping this, but for those more fortunate than me, the name alone fits for the season.
Spider Decorator Crab - it was already mentioned in OP with spider crabs, but I don't see the crab that dresses itself up specifically mentioned, you can do some fun stuff with making decorations to suit up for halloween, from 3D printed candy corn, giving it a trick-or-treat bucket for a photo, etc.
Now that I got a saltwater candidate out of the way; If it's allowed I'm going to suggest some stuff that isn't necessarily reef, saltwater, or what you'd traditionally see on these forums, these animals were just made for a halloween theme;
Halloween Moon Crab (Gecarcinus lateralis) - I believe they're terrestrial brackish water crabs that spend more time on the surface and need salt water to thrive long term, very fun critters if you're into paludarium setups. They get pretty big too, I used to have them but never did a the glorious setup they deserve, you can purchase them from some saltwater retailers, but they're commonly found from reptile vendors;
Vampire Crabs - Named for their bright eyes, and nocturnal activity, they're a smaller terrestrial crab that needs bodies of freshwater to submerge in. They're also very simple to breed, infact they're hatched out of the egg as functional crabs so no free swimming or sea water life cycle, you can build a thick planted paludarium and watch their numbers increase. Their popularity has been skyrocketing these past few years as shops realized how easy it is to build a colony of them.
