Is this fin rot?

[Cold Canadian]

Hello, a few of my Cardinal tetras have a beigh/yellow thing on its tail. I am not sure if it is fun rot or not. All of the fins look ok on all of the tetras. I also remember scooping out a dead tetra a month ago, it had been missing a tail fin.
All of these factors make me think fin rot but I am unsure because the fins are not rotting.
Current tank mates are endler live-bearers, bronze Cory's, Otto catfish ramshorn snails and RC shrimp. The tank is well planted, and has been set up for a few years now.
Advice on what it is and remides would be appreciated.
I hope the pictures work.

 

[coralbandit]

I can't tell what it is but it doesn't look like fin rot.
Does it move on the fish? Could be a louse[lice] or other large[ish] parasite.

 

[Cold Canadian]

No it is not moving on the fish. I'll try to get better pictures.

 

[Matt68005]

Cant tell either, dosent look like NTD or Tetrahymena, looks like lymphocistis but that shouldnt affect several fish.

 

[Kwakatoo]

Possibly fungus.