If you try to wash it off, it gets slimier as it gathers water. It is best to do your slug studies out of doors and leave them where you find them as they make a somewhat slimey animal for classroom studies and should not be kept in a forest floor terrarium unless you are willing to invest the time to clean the slime off the sides of the container. As well, make sure to handle them with clean hands as the salts, natural oils and other contaminants on your hands can damage their skin. Please do not kill slugs using salt. The thick goo you see on the slug after salt is put on the body is actually the fluid or blood of a slug that rushes to the skin's surface to dilute the salt.
Dehydration is a nasty way to go.
Simply remove the Banana Slug from your garden and place it back in a nearby forest where it can continue its important role as a forest floor decomposer . Take time to look for one of these fascinating one-footed invertebrates next time you are walking in the forests of the Pacific Northwest. Watch them do their important role as forest floor recyclers and then walk on, leaving them to do their job.