We couldn't identify it with complete confidence. It looks a lot like a Lunar Moth caterpillar, but not perfectly the same (based on Internet descriptions).
We hung out with him for awhile. I took a bazillion photos and was basically entranced. I loved watching him walk. I even videotaped him walking and then pooping (a bonus!). I'm sparing you that little video. Let's just say, we didn't know caterpillars pooped such big poopies (I just use a form of the word poop four times, aren't you happy?)!Such beauty. Seriously. What a creative Creator we have. After an hour, we let him go. We don't have the best track record with caterpillar to chrysalis to moth/butterfly experiments.