So it was sticking outside of the gills?
When you pulled it out, was it attached via any artery or anything?
I'm thinking since it's already out and likely you won't be able to put it back in/reattach (if it is an organ), might as well dissect it. Is it hard?
Anything in the tank that bears resemblance that the fish could have swallowed that?
Did you just buy the fish with this growth? Or was the growth/object not there before and suddenly appeared? Or grew slowly or rapidly?