I have a 10” Dat and every few months a small white cyst forms at the base of the tail stalk on one side. It starts out very small, grows to approx. 2 mm in diameter, sits around for about a week and then recedes. This has been going on for well over a year. The fish appears to be completely healthy.
My point is that you will see many such anomalies with fish and you always have to determine, based upon visual inspection, if the fish appears to be healthy. If so, and given that you can’t do anything about a tumor anyway (if it is a tumor to begin with), just forget about it. Sometimes, these things are part of the biology of the fish and we just don't understand their morphology and physiology well enough to know what is going on.