this is very easy to clear up , i have had this on 2 diffrent ferts in the last year .
all you need is AMOXICILIN. broad spectrum anibiotic , its used by doctors and vets , for wounds to the skin , and even dental problems.
first thing is get that fert on its own in another cage, then get 1 tablet of amoxy, somtimes there in capsuals. (capsuals can be opened and emptied out) if its in tablet form . crush it up in to a fine powder and put it in to a water drinking bottle. the type that you put on the outside of the cage.
now evry day when feeding the fert, crush half a tablet up