On all livers, including humans, there is a small sac, this is called the gall bladder. It is best to gut the fish and try not to break the sac as the contents are quite bitter, but should it leak the green stain it leaves can be washed off or trimmed from tainted flesh.

If your buddy likes to eat fish liver (I certainly do, along with fish roe) then it pays to take extra care, remove the liver whole then with careful use of the knife, remove the sac.

Should the sac burst it makes no difference to the fish whatsoever, it is still very edible.

Hope this helps.