You’d better wear some flowers.
Synchlora aerata-Wavy-lined Emerald moth larva. These actually don’t go to California but are found over most of eastern and central U.S. Here in the northeast they are the only species to cover themselves with small bits of flowers and other debris for camouflage. There is a southern species found elsewhere. There were several on the boneset and all seem to arch most of the time but this one appeared to be rearing. Maybe it was roaring.