It looks like you need to clean your PCB's after soldering!
The remaining solder joints looks ok, it's hard to determine if it's done on an assembly line - or if you have re-touched the IC by hand later.
No matter anyway, a good clean-up is needed before it gets to this state.
Getting the correct flux/soldering paste might do it, time will probably let you know - the bank might not like it though
Acids and salts can be present on a board, and can be almost invisible to the naked eye.
It can also be quite hard to clean, so use the recommended cleaner.
(And do clean the entire board, not just particular spots - the nasty stuff can and will move around, it's just a matter of time)
On a different note - it can also be moisture damage resulting from dried up saltwater. (It conducts and etches)
Do you have a kind of hot-spot, thermal vent hole or convection area moving large parts of air over that particular spot?
- not speaking of direct moisture settling/spraying as that should be a no-brainer