Black Moor Goldfish has large lump/growth on back near top fin, what could it be

I can't really help you diagnose as I can't see your moor but this are the few possibility you're looking at and you should read them up and assess for yourself. My best guess it that it could be Nodular disease but it usually don't get to grape size.

Another site you can try is http://www.provet.co.uk/fish/spotsonfish...

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Seen the video. I now believe that the lump is due to a genetic problem. The fish looks healthy and not in distress, which support this theory. The lump reminds me of the wen of the goldfish (the lumpy flesh on the head of lionhead or oranda goldfish), when the wen gets injured, the healing wen will look whitish translucent and then turn into the colour of the fish ( black in your case ). This is the first time I have seen this and its an abnormality which no medication is going to solve for you. You should post this on a few of the specialized goldfish website and tell them what I think to get a second opinion. I believe it will interest people that studies genetics of goldfish.

Anyway, back to the topic. Currently the moor look fine, but if the lump continue to grow, it's will have problem swimming upright which is bad. If you can bring it to a vet who's train in fishes, he should be able to slice it off. As long as anti-biotic is applied and water is kept clean, the wound will heal. The bad news is, if this is genetic as I predicted the lump will continue to grow out again. But seek your vet advise.