Try taking a damp wash cloth and wipe the area to see if the color comes off. I expect it will.
Do you have any bird toys in the cage? These often have rawhide or wooden parts that are colored with food grade dye than can come off when wet.
Look for metal parts of toys like chains or pear shaped hooks. These metal parts often turn green when they come in contact with glider pee - much like costume jewelry will turn your skin green after coming in contact with sweat.
He probably has just rubbed against something while marking the cage and gotten this on his fur.