Neither can I say I've seen this but for now, IMHO, this is a rather unremarkable imperfection, looks like a healthy tissue, a large wart of sort... still unusual I agree. I'd wait it out and see what time shows.
Aren't warts in humans at least (not warthogs

) caused by a pathogen though? If so, what kind? I am having a vague recollection that this was proposed / thought of at some point by the medical community, perhaps decades ago; don't know the current state of knowledge on warts.
Stunning fish! Is this the very expensive lung from Wes?
[1] If very worried and [2] a precedent can be found that flesh-dwelling worms can do that right under skin (my impression is they prefer to reside deeper in tissue) and [3] praziquantel has no effect, which it seems like it is not, I'd try fenbendazole, with feed as you were doing.