So this first started happening in my cheap acoustic about a two or so years ago, but I've recently noticed its happening on my electrics but never really cared to much to do anything about it. After new strings have been on my guitar for about 4 days to a week, depending on how long I play for they get this green mouldy/dirt type thing sitting under the bottom of the strings, it looks and feels gross and it leaves my finders all dirty, I cant say if it affects the playing as I live far away from any other guitar playing friends or guitar shops but I have noticed bar chords are really hard to hit without buzzing but that could just be me. I've tried multiple gauges, elixers, daddarios and ernie balls so far and they've all had the same problem. Any one know what's wrong or how I can remedy it?

For your troubles have some Abalone
Where are you keeping your guitar? You might have an issue where either a strange reaction is occuring perhaps due to a strange moisture/humidity problem. (which could also be the cause of changing your neck making your barre-chords harder to play)

Check to see if your neck is warped and make adjustments or bring it to a guitar mechanic if it is not properly straight.

Another possible thing could be that your fretboard itself is dirty, so whenever you play or do bends the grit/grime from the fretboard may be getting onto your strings. The next time you string purchase some Dunlop Fretboard lemon oil and and wipe your fretboard down good!

Also, for safety measures, wipe down your strings after every time you play.
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Happens to me all the time. I'm pretty sure it's the oils on your skin reacting to the metal. It shouldn't affect your playing and it won't damage the strings in any way, it's merely just a build up of gunk that comes off your fingers when you play. Changing string gauges or brands isn't going to help either.

Keep a cloth or a rag or something like that where you play and wipe down your strings after you finish playing.
Keep a cloth or a rag or something like that where you play and wipe down your strings after you finish playing.[/QUOTE"]
and im in a humid/tropical climate so that could also be it, normally its just on its stand

thanks heaps