Not the Black Moor goldfish but the disgusting white sores on its side.
Ichthyophthirius multifiliis is a common problem for aquarium fish. Ich is a species of protozoa that infest fish’s gills and skin. In the wild it’s usually not serious but, in small bodies of water such as aquariums, it is often fatal.
I hope this outbreak was caught in time. The issue with the heating and filtration killed hundreds of fish. This is putting me off aquariums for good.