CCUK
Free Flying
You should be OK to just wipe vaseline all over the legs. All you need to do is smoother the legs and the mite will die. The two suggested by @KikisGirls may well be cleaner and easier to administer as it is a spray rather than a thick oily cream than can get quite messy especially when the hen's are scratching around. You can also use ivermectin but you will have a egg withdrawal period.