This spectacular green spiky caterpillar is the larval form of the spiny oak slug moth. It’s probably Euclea delpihinii, based on comparison to photos on bugguide.net, but there are two virtually identical species of Euclea in my area and this may well be Euclea nanina. I am not yet able to afford the $30,000 reference library to determine for sure, so make of this all what you will.
The adult moth of E. delphiini is furry and brown with two bright green patches, one on each wing, which can range from tiny to almost covering the entire wing. The larvae, as you may guess from the explosion of “Don’t touch me!” color, are venomous, producing reactions ranging from itching to burning to anaphylactic shock. The color patterns and number of spines vary widely, both within and between Euclea species, but E. delphiini, particularly, seems to generally have six big spines at the front and four big spines at the back (hence my guess at identification).
This is also a “slug caterpillar“, a type of caterpillar with a fleshy “skirt” covering the legs (and the head, when it tucks it down), concealing the caterpillar’s underside and possibly providing some protection. You can see that “skirt” very well in the following photos:
The tail and head end are hard to distinguish when the caterpillar hunkers down and puts all its spines up. Later instars of this caterpillar are supposed to have 2-4 black spines, called caltrop spines, which can break off, but I don’t think that’s what the black spots in the below right photo are. (I can’t find photos that specifically say “these are the caltrop spines”, unfortunately.) Head on the left, tail on the right: