That is probably bumblefoot. Some chickens get it, and it doesn't become a problem, but it is is red, swollen, or the chicken is limping, it needs to be taken care of, usually with the surgery to remove the pus core. I have a lot of tiny sharp rocks in my soil around the coop, and many have those bumblefoot spots. I do the surgery if it is bad looking, but sometimes they don't pose a problem. You should look at the foot periodically to see if it needs to come out. Some people have used epsom salt soaks, or a product called Tricide Neo to treat it, and some give penicillin, but surgery if the quickest cure. Here is a good link with a video of the surgery:
http://www.the-chicken-chick.com/2011/07/bumblefoot-causes-treatment-warning.html