This looks like columnaris to me (aka 'mouth fungus', 'cotton mouth'). It's actually a bacterial infection. It's generally brought on by stress, poorly kept aquaria, or recently moved fish. To my knowledge, it can be quite contagious, but doesn't always progress quickly.
These are some questions to think about:
Is this a new fish?
Any stress or injury that might have occurred?
Do you clean your aquarium regularly?
How long have you noticed the signs of the infection? The first signs are often discolouration around the mouth, but this can be easily overlooked until the cottony patches and lesions appear.
It's possible that this is a secondary infection. That is to say, the fish injured his mouth and the bacterial infection is a result of it. In any case, the causes are the same.
To treat the infection, I recommend an anti-bacterial for external infections. I would also do a water change beforehand. I'm sorry, but I can't recommend a name brand in particular, as I haven't used any in the UK yet.
You
can treat with salt, but since you *probably* have a planted tank anyway, that's not the best route. Treatment may or may not save this particular fish, but it should keep it from spreading to your other fish. This all depends on how badly the fish is infected (whether or not it has spread internally or how starved/ill he is from it).
Best of luck
EDIT: Just to add on medication, you apparently can get API's Melafix at Pets at Home. This has and also hasn't worked for me in the past. I know aquarists who swear by it and who think it's rubbish. It's tea tree oil and has anti-bacterial properties, so it can work for some things...just something to think about.