Can anyone identify if this is a parasite?

Hey guys...
I've had my otos for around 3 months now and even when I first got them I noticed a small discoloured spot on the underside towards the tail end of one of them and he/she seemed fine so thought it might have been scar tissue. I then thought it may be eggs but now I don't think so as one of the other otos appears egg laden and it doesn't look the same.

I've been keeping an eye and noticed today the mark looks bigger and kinda worm shaped/C shaped, and I'm concerned it might be a parasite/worm buried in its skin/flesh as it doesn't appear to be in the main cavity of the body where the organs/digestive tract is.

I managed to get a photo and was hoping someone might be able to tell me what it is, as I want to act on it before it becomes a problem if it is one. I have been searching online for everything and can't seem to find any thing to match what it looks like. If it's not obvious, it's the yellowy coloured mark near the base of its tail if ypu will.

Thanks in advance.
Amy

It could possibly be a tumor: @Colin_T

possibly a parasitic worm that got lost and now lives there. If it is, you can try deworming the fish or leave it. If it isn't getting bigger, then leave it.