Generally it clears on it’s own, although I have one common in my pond that seems to struggle to shift it and has had a small bit on it’s lip for over a year, albeit smaller than it was. I did keep saying I’d dispatch it but I can’t bring myself to, it’s a struggle to get much of any fish here for reasonable prices now (and the choice is rubbish!) let alone get hold of commons and mirrors!
I’m no expert but it tends to appear in my pond when the fish have been distressed for whatever reason. It could just be the cold temps and hopefully it’ll clear for you but it may be worth checking your water etc, just to make sure.