Are you sure it's a growth, or could it be an ulceration? It's not unusual for a cory to get red blotches on them as one of the many possible symptoms of a bacterial infection. This is the most common thing that makes a cory sick, and while the corys are often the first to show symptoms, if this is the problem, it could also effect your other fish.
If this could be the case, give your tank a good water change and bottom vacuuming and use either Waterlife's Myxazin or Interpet's Anti Finrot and Fungus medicine. It's best to isolate it if you have a hospital tank and keep the temperature no higher than 24 degrees C. Make sure that the water is well aerated and add an airstone if necessary.