Never seen anything like it. Have you looked up anchor worms ? One of the few things I can think of that protrudes, but I've never seen a case in person.
Does it appear to bother him ? Is he eating, swimming, etc. or showing signs of distress ?
Might possibly be some sort of tumour, if so, it appears to be attached only at one end, and you might be able to remove it. Clove oil is a good fish anesthetic, provided reasonable care is taken with dosage and timing. It's used for euthanasia too, yes, because overdoses kill, but it's the most common fish anesthetic, and can be used safely for minor surgery and treatments.
Wish I could be more help, I can understand the concern.
