That looks like over-application of old-fashioned Cosmoline industrial lubricant to me. If the wood isn't soft, don't worry about it. Unsightly, yes. Structurally significant? No.
I respectfully disagree with one point above: DON'T use steel wool. The steel fibers will get into the electronics and foul up everything worse than the neck pocket discoloration. A small strip of very fine sandpaper wrapped around a very small tool to carefully see if it is just a surface stain issue. If it has penetrated, you have to live with it because it will involve removing too much wood to clean it up, compromising the neck pocket.
Since you have six neck screws, it is definitely not a structural issue. Three will hold a bass neck. ('70's Fender Jazz Basses). Four is standard; five is overkill; six is surviving armeggedon.