Not an expert... It looks like only the trim has been pulled back so far. If it were me, I'm definitely taking out the toilet and existing flooring to address the underlying sub floor and see where it's coming from and how far it spreads. Ideally what you'll just need to do is pull up and replace an almost paper-thin sub, cut out and replace a small patch of drywall behind the toilet, and clean with bleach and apply killz in various places. That might sound bad but if that's all it is you're looking at a day of work and a fairly reasonable shopping list: sheet of sub floor, some nails, bleach, Killz, patch of drywall, some mud, some tape.