Is this safe?

[SJRSJR]

Was connecting a new wall socket and I see that the ring main earth wire is a bit green and corroded looking. Never come across it before. I've cut it back but six inches in it's still the same. The cable is in a void under the floorboards. There's plenty of slack so I could cut it more but what if it's like that all the way? Is it dangerous? I would replace it but it goes through the wall into another room with a tiled floor and kitchen cupboards so it would be a nightmare to do.
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[AndyPRK]

Use a pan cleaner to clean it up to shiny copper

 

[flameport]

Green and slightly corroded = has got damp at some point. Not a problem.

Green and also oily / wet / slimy = cable needs to be replaced.

 

[SJRSJR]

Yes, it's about three feet from a damp exterior wall (where builders had bridged the damp course by dumping all their rubble in the cavity - since sorted). That's a relief, thanks.