From what I can see it looks like a clump of mud and/or poop. It is normal in chickens for this to happen sometimes, Silkies are especially prone to it. Bath it in some Epsom salts to see if it loosens. If it does try and wipe us much of it off his feathers as possible using an old towel, or dishcloth. If it all fails it’s time to bring out the scissors. You’d need to cut those feathers off his feet. It may seem harsh but it would be the best thing to do. After you’ve cleaned off his foot check and see if you can see any cuts, or bumble foot. If he is still limping it could be sprained. He is probably limping because he finds it feels weird to walk on that foot. It’s best not to rule out all possibilities though.
