Is this a pest?
The caterpillar blends in with the background, but I believe this is one of the cabbage worms.
Here are the descriptions:
You'll want to watch for little butterflies/moths to get a better idea of what it is. Basically, the adult butterfly or moth is laying eggs and these caterpillars are eating holes in the leaves.
There are a variety of control options from physical to cultural and more listed on the factsheet link above.
They are indeed a bad insect for leafy greens and cabbage plants!