Gross! Time to deworm your whole flock (not a bad idea to do the dogs and cats if you have any of those too). If I'm not mistaken, once the worms show up in the poop it usually means they are carrying a fairly heavy worm load. Your best bet is to use something that will kill them gradually so that they don't get dead worm overload. See this link https://www.backyardchickens.com/forum/viewtopic.php?id=517234&p=1 for an excellent general chicken illness overview. Worms are at the bottom of the first post. Good luck!