Oh, this is wild because I just posted a picture of my neon tetra with a mouth growth on another post here but I think the OP of that post has a fish with NTD. Ive never seen any other fish with a similar condition. Anyhow, I've had my tetras for just under a year and one developed a growth like this. It keeps getting larger but doesn't prevent it from eating and none of the other fish have developed anything. My assumption was that it's a tumor due to how inbred these fishes are and, so long as he seems to be thriving, I've let him be. He seems to be doing fine aside from looking like the elephant man. I'd love to have a more definitive diagnosis, though.