I'm not sure about the 'mask.' I know EXACTLY what you're talking about, but I've always attributed it to different frog species being lumped into the common name of 'African Dwarf Frog.' ONE of my females looked like that - ALL of my other frogs, both male and female have looked slightly different, so I think they may be a different type of frog altogether. I could be totally wrong. . . I *think* that the three frogs I have now are Hymenochirus boettgeri, while my original female may have been Hymenochirus curtipes. I'm REALLY not sure about this, though. I've tried looking into it, but can't really find any information that concretely defines the differences between these two, and the other types of frogs sold under the common name. And I've looked! Almost everything online, even when they are given their proper names, interchanges the types, and I've never been able to track down any good scientific papers or books on the subject. I may have to see if I can ask Stuart to clarify. He wrote the Wiki articles, and keeps several species. I'm betting he would know. . .
lol, sorry for the babble. Something I've always wondered about, obviously! If you have any info, I'd love to give it a read ^.^