It's a tell-tale indicator. Warning that the mechanical water pump seal is leaking.

If gearbox oil was coming out of it, it would mean the water pump oil seal is leaking.

Unfortunately, you'll have to take the side cover off to replace the seals. When I do it, I change both of the seals and the small bearing too.

The mechanical seal is around £45 inc vat. I used Stephenrc45's advice and pressed the old one out and the new one in using my bench vice.

You have to tap the bearing out and remove the oil seal first though.