It didn't take long for the caterpillar to resume eating. My hunch is that I caught it on its last day out. Imperial moth caterpillars, after growing to maximum size and maturity, leave the tree and go to the ground. Once on terra firma, the caterpillar finds a suitable crevice, hole, or spot of soft soil and heads below ground to spend the winter as a pupa. It'll emerge as an adult sometime next summer.
Note the maple foliage in the background. As the days grow shorter, chlorophyll productions begins to shut down, and its attendant green coloration bleeds from the leaves. The underlying yellows, reds, and oranges are unmasked, and in maples, the leaves become yellowish, heavily dappled with maroon. Later they can briefly change to brilliant orange or crimson.
Ironically, for all of the eating that the caterpillar does over the weeks that it scavenges the trees, the moth eats nothing at all. Silk moths such as the imperial moth have no functional mouthparts, and live only to find a mate, copulate, and lay eggs.