Weird blister looking thing on my koi

[CaitKoiPond]

Does anyone know what this could be?
The fish has been acting very weird hardly swimming and resting a lot of the time on a ledge in the shallow end exactly as pictured.
Seems to have this weird blister at the bottom of its tail and also some whiteness on the tail. There is some past scarring from fin rot on the top fin but this has been there a while.
Pond has been tested and everything is fine except the Carbonate hardness was very slightly high. All other fish completely fine

 

[CaitKoiPond]

 

[CaitKoiPond]

 

[j.w]

@CaitKoiPond
I'm not good on diseases for fish sorry. People here talk about Koipox being one disease they get. It has lumps that show up and then go away after awhile. No treatment tho I believe.