Most of our slugs in the eastern US (maybe western too?) are exotic (primarily introduced from Europe). Among our native slugs, Mantleslugs are fairly distinctive. They can be readily identified by the mantel extending the length of the body (rather than being restricted to the anterior) hence their common name "Mantleslugs," and the mottled dorsum. There are two genera in your area that look like your slug: Philomycus (4 spp.), and Megapallifera (with one species: M. mutabilis). Though I've never seen this species, I think yours is M. mutabilis, based on the mottled dorsum with no distinct longitudinal lines. Philomycus species have longitudinal stripes (or spots in one species). Check out this key for more specifics: http://www.carnegiemnh.org/mollusks/palandsnails/key.htm

aww yours is just a baby. Check this out...