What worm/caterpillar is eating plant
This looks like the larvae of the dogwood sawfly, eating the leaves of a red twig dogwood. This late in the season, it is not of great concern.
It doesn't necessarily happen every year. You may never have the problem again.
Keep an eye out for them next year. You can pick them off into a bucket of soapy water, or spray them when they are small with horticultural oil.