Arion Red Slug

I think it should stay in your mission. They've been in the States for a fairly long time I think, and most certainly aren't just temporary visitors! The ones we have in Oregon tend to be darker orange, and some are downright black. The distinctive characteristic is how they hump up and hunker down. The banana slug doesn't tend to do that (plus the banana slug doesn't have those ridges). There are, however, a LOT of different slug species so a more accurate ID may yet come along.

This is not likely a banana slug--they don't have the ridges like this guy has. This is closer to Arion ater, maybe one of its color variants.