It's heated up and cooled grease. usually it hits the ball return belts and makes a hell of a mess. Either from a messy mechanic when they lubed the equipment or a seal from a joint or motor is leaking.
We have two pair that are like that. I replaced all the seals, motors and belts last season and it is still an issue. My hunch, which the owner disagrees with, is there is a build up in the subway system of the return.
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