your picture is blurry so this is an educated guess.. if its acid damage from the previous mpu it will keep coming back until you fix it ( by removing the affected wire and replacing the connector. You can try washing it off with vinegar but if its into the end of the wire its too late

Some have theorized it's supposed to be a conductive agent.
http://www.pinwiki.com/wiki/index.php?title=Bally/Stern#So.2C_Just_What_is_this_Green_Slime.3F

You are really going to have to replace a few of the pins anyhow. so you might as well change them all. But at the very least soak the whole connector in vinegar for a couple minutes while scrubbing with an old toothbrush, then rinse with a bath of water, then rinse bath of IPA and blow dry. Kind of a crap shoot though, especially connecting to a new board. Why risk it.

It is the plasticizers leaching out of the old wire insulation, usually any place it got warm for an extended period.

Don't crud up your new Alltek, buy some new connectors and crimp them on

This is the answer in my opinion. Just replace the connector. Snip it back to clean wire and remove the issue. Plus it is an old Bally, so it was probably due for connectors anyway.

Some of the green and black wire insulation coats break down to green slime. I see it in bally and stern games. It actually doesnt seem to effect continuity too much on signal plugs.

If you cut the wire back and recrimp the slime comes back after a period of time.

You can clean the inside of the connector with a pipe cleaner and Simple Green (or similar). It make take a few attempts to cut through it, but it will eventually clean up.