Thanks flori, that is it. He is pretty, but my tree isn't doing well so I am going to move him to a tree by the field.

Google is your friend. I typed in "Identify caterpillar" and found this real easy guide with checkboxes:

https://www.discoverlife.org/mp/20q?guide=Caterpillars

Color = green, hair = no, features = knobs.

The second one on the results list looks like this:

https://www.discoverlife.org/mp/20q?search=Hyalophora+cecropia&mobile=close&flags=glean:

Cecropia Moth which I think is also called the Luna Moth due to moon shapes on its wings. Big beautiful fella. I will leave it to you to determine whether the cecropia moth larva will damage whatever plant it is on. But caterpillars eat plants, that's what they do. I'm not suggesting you kill for that reason alone though.

See if you can find an appropriate host plant for him. Cuz there's nothing better than a moth as big as your face. :P

I moved him to new digs.