I dont know anything about phlalate plasticizer leaching out, but most figures that show these kind off "frosting" mostly have signs on the cardbacks to showing it has been wet or stored in a damp room.
If it was in a damp room with the right tempurature with it (there are many different kind off moulds and some grow in a damp space at room temperature) it could eaasily grow a mold inside the bubble.
Also the sunlight worked in many cases i've seen before. I tried with an UV light to get rid off mold once, but that did not work. I talked to others about this and it is the combination off the sunlight and heat that does the trick.