Any ideas what's going on here?
I can only come up with gases escaping from screw cap and reacting with clamp.
But does not explain how it has ended up "sprayed" elsewhere
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That's a bit of galvanic corrosion. I suggest removing the clamp, cleaning it all up, and giving the clamp a spray of paint.
That was my option A.
Time was moving on. Cleaned up with a brass brush. Some bare metal. Smeared some battery terminal grease over the bare metal.