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
You cannot view this attachment.
You cannot view this attachment.
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.