It's copper patina that forms on any copper lame fabric. To keep it from forming, you need to wash the the mask regularly to get rid of the sweat salts that encourage the formation of the oxidation-- mild detergent (Ivory Snow or Woolite) and ammonia works best. If oxidation has already built up, leaving the mask to soak for a spell in the detergent/ammonia solution may clear it up enough to pass the conductivity test (or at least be useable for practice). Just making sure you rinse or wipe the mask off with water after each practice or tournament will also help a lot. Spraying the bib with Windex (or another ammonia-based cleaner) and wiping it off with a damp cloth is another method to do a quick cleaning.
Note that the fabric backing will usually remain stained green-- that's no problem. The Blue Gauntlet copper lame material in particular will often develop a lot of green coloring in the fabric while still conducting fine. What you want to do is look at the metal fibres to make sure they're still clean. If the metal is completely corroded, then you'll need to put a patch of good lame material over the bad spot.
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agreed, treat it as a lame, if it is dead, patch it, no need to worry about the mesh as much though, just scrub it with a stiff old toothbrush (not that you should need to)