From your picture you have cabbage worm caterpillars from the cabbage butterfly. This pest is very common in brassicas. There are 2-4 generations per year. You will see the small white moth flitting around as you become more aware of its presence.
You can easily handpick the caterpillars and dispose of them in soapy water. The caterpillars are tasty for birds and other predators. Bacillus thuringensis (Bt) is effective on young caterpillars, and you can also use neem oil, spinosad, or pyrethrums. This article has pictures of the various life stages and information about organic control options, Imported Cabbage Worm https://extension.umd.edu/hgic/topics/imported-cabbageworm-vegetables.
Cabbage butterflies overwinter as pupae or chrysalis in plant debris and emerge in spring. Removing plant debris in the fall can help control this. Using row cover can prevent the butterflies from laying eggs on developing plants.