It looks like lymphocystis to me. I would do a search on it and check out some pictures to compare. It's not necessarily fatal.
This is a bit steve-s wrote on it, hope it helps:
I copied this small part from an article I wrote for another forum but in a nut shell...
Lymphocystis is a benign dermatotropic affliction caused by an iridovirus. Lymphocystis typically confines itself to skin and fin areas which superficially affect the health of fish. Although unsightly, it is not life-threatening unless it interferes with proper gill function or grows over the mouth opening.
.
Usually the result of poor water conditions, Lymphocystis can quickly become a scourge on the affected fish unless steps are taken to alleviate the condition. There is no medicinal cure for Lymphocystis but it is easily treated otherwise. Commonly, not all fish within the same system will become infected unless living conditions are quite substandard. In most cases, it will be confined to one or two fish, depending on stock levels. There is no need to quarantine the fish otherwise.
The best prevention is a clean, healthy, stressless environment and proper feeding routine of highly nutritious and varied food types.
