Article 'What to do when your flock is struck by FLYSTRIKE!' https://www.backyardchickens.com/articles/what-to-do-when-your-flock-is-struck-by-flystrike.78243/
Those are maggots
This is my go to, it's a 1 and done, no need to repeat treatment. I recommend mixing up about 3 gallons in a 5 gallon bucket and giving everyone a dip(as long as your weather is nice, if it's not a spray bottle is fine, be sure to hit the vent, under the wings, and back of the neck well), then...