That looks like scale mite and you can treat it with vegatable oil and all you have to do is brush it on and while you are at it I would treat the whole flock witht he oil . The mites dont bother the chickens but eventualyy will cripple them as the mites eat on the scales of the legs and cause them to lift and get hard. I got some that had them and treated them with veg. oil...all gone and not much work either. yep your best bet is to try to catch him at nite. Good Luck