Welcome to the forum and congrats on the new-to-you tractor.
That milky fluid will need changed for sure.
If the tractor sat for a long time, it can accumulate moisture, especially if it was stored in a location with lots of temperature changes. Also check fill cap and vent tube to make sure they're not in a position that could collect water. (e.g. if your vent tube points up but the hose is gone or cracked, rain can get in that way.
I'd change the fluid then watch it for the next couple months to see if it reoccurs.