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Wear matching sweats, tights, or skirt. Cut strips of fake kelp. Make a comfortable backup costume (optional).
If you can, make your whole costume the same color. A black plastic garbage bag makes convincing seaweed when cut into strips. Drape it over your tentacles or stick it in your belt to hang around you. If you like, cut a secondhand skirt into eight pointed strips to further mimic tentacles. This is a less striking but more comfortable costume you can change into if the larger tentacles start to annoy you.