I must start with referring to the GI having a Bridge Rectifier related to it, my apologies, I was wrong and thinking of the Switched Illumination.
I have just had exactly the same thing in a Bally Vector, the green slime on one pin.
I believe it to be the 'breakdown' of the cable insulation itself.
I used a cotton bud with methylated spirits, an alcohol.
If you are unable to remove the pin that is.
And also a small stiff haired paint brush or baby screwdriver with a rag on it to get in and around the pin.
But, that won't be causing the fuse to blow, but still should be cleaned up IMO.
Also just my opinion, but I see the four 100ohm in parallel resistors to give you your 25ohms, I'd be swapping it out and the 600ohm and raising them off the board, but this isn't necessary for your game to work.
As your GI has proved good, I'd be plugging the connectors onto the power supply board one at a time to see what part of pinball is causing your 20A GI fuse to blow.
A pic of the reconditioned power supply