My fish looks sick -advice please

Hi guys,

One of my fish looks sick and I wondered if anyone could give me a clue as
to what is wrong with it please.

It is a 2 year old Shubunkin and it seems perfectly normal (it is eating and
swimming just fine) but it has what looks like a large sugarcube size square
of fur growing on one of it's side just under the Dorsal Fin.

I've looked at some fish health sites but can't seem to match my fish's
illness to any of the ones described.

Can anyone tell me what it is please?

Thanks in advance for any suggestions
Jay :-(