It's some kind of as yet unknown infection that only NTT's seem to get.I've had a few,on different occasions get those lumps.Sometimes they go away and sometimes they get bigger and the fish eventually dies.
Kind of hard since we don't know what it is and what causes it.I have an old thread in which I talked about it happening to my last NTT.I lanced the lump with a needle a few times and it oozed a whitish puss looking substance.Its one of the reasons I try to stay away from thinbars....as beautiful as they are.They always seem to suffer from some ailment or another.
Pretty much and don't be surprised if another or a few more lumps appear.I'm on my phone now but search my old threads from a year or so ago and read the discussions me and a few others had about it.
I had that on my large clown loach, it is from dirty water. I did a water change and use a razor blade and cut it off and rubbed area with salt. It cure up within 3 days after that.