if it's not rust it's probably something that has spilled on there. try
a stronger solvent or scraping with a razor blade.
but cast iron is pretty porous, and it does rust beneath the surface.
this is likely what you are seeing. you can try something like
barkeeper's friend, which will likely get the stain out, but with time
the whole thing will darken a bit. it's not a problem. just keep it
clean and use it.