Thanks for posting though, at least I’m not the only one!
Looks like it needs a little Armorall wiping on it or some other silicon type plastic treatment.
Nothing is impossible for the man who doesn't have to do it himself - A. H. Weiler
Could it be rising damp and the pack is getting mouldy ? (given its warm and will inhibit all sorts growth).
I tired clean it with my hand but those things doesn't come off easily. I'll try damp cloth then.
And since OP still using it fine after a year I guess it won't catch fire... Well, guess I'll have to live on with it. Thank you all guys.
I would say it's mould, like Goosey suggests.
Let's see - surface temp is 33°C, relative humidity is 48%, and no other power adapters nearby have this issue.
No, it ain't mould.