Couldn't find the exact thread but I think it looks like an anchor worm, which I have seen in threads only, but couldn't remember the name. I found this old post by Humblefish "anchor worm - treatable with Dimilin"
I know humblefish spends a great deal of time in the fish disease forum and IMO FWIW he is very knowledgeable. Might be worth sending him a PM to see if that clicks with him.