Hmmmm... not too sure what that is tbh, could be a bacterial infection or worse case scenario a type of tumor...
Im leaning more towards an infection myself based on the once a month water changes, which could have happened if the puffers got in a bit of a scrap between each other and water quality during that time deteriorated just enough causing the problem? Especially since they can be quite sensitive to any dips in water quality being scaless fish...
I also see that you said you’ve started a round of dosing meds already, so keep an eye on him and see how it goes...
as long as they are still eating and pooping okay that is a good sign but if it gets worse or conditions dont change i would probably quarantine the infected puffer for peace of mind and to prevent any further damage to that puffer or any others in the tank...