Is it a finished neck? I presume so. I don't know the answer, sorry, but a closer inspection should reveal what has happened. A look at the pic seems to show a neck with a lot of wear, I can see wear marks on the frets even on that scale. What are they like? If they are very worn you will be up for a refret.
I wonder if the stain is where the original finish has worn through, allowing the players fingers to mark the fretboard with sweat and dirt. I'd avoid using anything with water in that case, as you don't want damp in the wood. Although maple is very dense so the water penetration would be low.
I have no idea. I might try wax and grease remover. But I'd guess you will have to remove all of the finish from the fretboard, possibly lightly sand it and reclear it for it to be gone.
I'd be interested to hear what people in the know might say.....
Last word... I like the aged look of the neck as it is!