He is a crustacean, related to lobsters and shrimp, that is a nuisance but does beneficial things if it would stay outside.
They do not carry diseases to man, animals, or plants. They are not poisonous but they do have glands that produce a terrible odor to repel preditors. I recommend that you wash your hands after picking up the dead specie.
They venture from their moist habitat at night to feed on leaf litter and other decaying matter. They are subterranean during the day performing aeration of the soil. They are very suceptible to desication if they travel far from a moisture source and is why people mainly see them dead inside. They enter homes primarily through weepholes in the brick veneer or any crack or crevice they can find. They don’t know the difference from upstairs or downstairs and can be seen in either. They eventually enter into the living quarters through carpet edge or plumbing chases or even canned lighting. Greenhouses are notorious for millipede infestations so new plants indoors or out are a highly possible origins.
Regardless of the control measure you use there will still be some that penetrate through your insecticidal defense. Block their avenues with copper mesh in the weep holes and other crack or crevices. I like silica as an insecticide because it is a drying agent and quickly kills the bug. In the mulch beds against the house I like Dylox an organo phosphate still available to PCO.