Thanks for the question. These appear to be cabbage aphids (Brevicoryne brassicae). These nasty critters can explode in population. Spraying them off with water can reduce populations but when populations are large it may take a bigger hammer such as insecticidal soap, neem oil, horticultural oil, etc. Catching them early is important. Check out the following links for more information:

• https://www.vegedge.umn.edu/pest-profiles/pests/cabbage-aphids
• https://extension.unh.edu/blog/start-scouting-now-cabbage-aphid
• https://ag.umass.edu/vegetable/fact-sheets/aphid-cabbage

I hope this is helpful,