This article indicates that it's probably gall bladder bile:
The gall bladder [...] sometimes [...] breaks even when you are cleaning the fish very carefully and close to the skin. If the gall bladder is broken, the greenish yellow biliary fluid pours immediately out into the inside of the fish and starts to absorb into the meat. The meat becomes bitter.
[...] see if the fish is greenish yellow on the inside or not. If it is, then quickly take a piece of paper towel and wipe it off; if it has already started absorbing into the meat, use a filet knife to cut off the part of meat that has turned yellow.
So, it looks like the gall bladder broke and stained some of the meat. If this is the case, cut the yellow bits away, and the rest of the flesh should be fine.