is it on one foot only or are all the feet affected? my chinchi had something similar that developed into sort of hard skin patches and while i didn't have wire mesh floor as such - and he didn't really spent that much time at the bottom of the cage), the vet advised to cover his shelves and bottom with fleece (as well as giving him a gel he absolutely hated). the problem went away and since then all my chins have fleece pads at the bottom of the cages as well as fleece tunnels and snugs etc.
i think it's prolonged exposure to hard surfaces - if there's no option of soft bits, the skin on the feet gets a little damaged.
there's a condition called bumblefoot which affects feet (mostly exposed to wire mesh) but i don't think that's your case.
