Hi,
It will be some kind of oxidation.
I use Weller, too, and did not experience this.
So either your soldering iron uses a different suface material, or the surface is scratched, or it's just an environment problem.
To remove it: Avoid scratching the surface, avoid strong acid / alkaline ...
Water, maybe a piece of plastics (cold iron!).
Check whether it's caused by the tip, the heater or the combination of both. (electrochemical problem)
BTW: I use distilled water on the sponge and I avoid cleaning the tip with steel wool (even if gold colored)...to not hurt the protection layer.
Once the protection layer is hurt you get those nasty tin spikes. Then the tip should be replaced.
My tips usually last way more than a year..
Klaus