There is a gear reduction case behind the engine that is filled with hydraulic fluid.
Your leak is likely the output shaft seal of that gear case.
The tractor has to be at least partially "split" at that point and the 2 halves rolled apart to remove the driveshaft to replace that seal.
Commonly grass will get wrapped around that seal and cause it to leak or damage it. If that is the case I've had some success just carefully cleaning out the seal where it seal against the shaft. But you take a serious risk of damaging that seal if you try cleaning it.
It is not a difficult job but is time consuming.
If you do take it on yourself I would recommend getting a workshop manual. It goes through the process in reasonable detail.
There is also a plug just above that seal that I would replace at the same time and you may want some new o-rings for various hydraulic lines.
My recommendation would be to leave it alone until it annoys you enough to spend the time or money to fix it.
I've seen them have a minor leak like that for many years and not get worse enough to be worth fixing.