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Hm, I'm honestly not sure. I would just keep an eye on it and if it starts growing or changing in any way I would pick up an herbal anti-fungal /bacterial medication to treat whatever it is. Doesn't sound malicious though.
A picture of the spot would be helpful. What size is your tank? In the meantime be sure to keep up the water changes with dechlorinated, temperature matched water.
You are going to have to get a larger tank if you want your goldfish to survive. 1.5 gallons is far, far too small. You need at least a 20 gallon tank and good filtration.