it's a mixture of sweat and dirt. it happens on nitro'd guitars. my AV strat and Les Paul have it all over the neck and elbow area after i've been playing them all day, and need polishing. the scum doesn't harm the finish as far as i know, as i've gone months without polishing my AV and it still comes straight off, finish is fine, feel is fine, etc... they say you can use good furniture polish in the AV pamphlets, but if you have any doubt buy a can of gibson polish, because gibsons are nitro'd.
