It's a 5th instar nymph. From the habitus I would guess most likely some Coreidae in the vicinity of Gonocerini ?? However, I found no data on any of those being registered as "pests" on Walnut, so I may be wrong and it might still be a Pentatomidae with a - to me - somewhat unusual habitus.
Best you can do is keep one or a few in a Jar, and feed it/them leafs (insert some mite invested leafs also, as it might be a predator!) and wait for it to moult into adulthood which would certainly improve the chances of a proper ID.