As far as I can tell, this is a banana slug. It appears that they are the only type of slug that is native to this area. They are often yellow. I sure hope we find a yellow one sometime. Here's a quote from Wikipedia about banana slugs:
Banana slugs are detritivores, or decomposers. They process leaves, animal droppings, and dead plant material, and then recycle them into soil. They seem to have a fondness for mushrooms, and they spread seeds and spores when they eat. They move relatively slowly since banana slugs, like all slugs, have only one muscular foot.