That is either the exhaust cover gasket leaking on the back of the motor between the cylinder heads or the powerhead base gasket that attaches to the midsection.
Take a deep look around your powerhead and see where the seepage is.
How big of a deal is it, not much of an issue IF the cooling passages are not compromised.
Otherwise that gasket has to be replaced if you have cooling issues or moisture in clyinders.
My guess is the exhaust cover gaskets. With a flat back cover, I think it should be possible to remove using socket swivels without pulling powerhead or without removing Thermostat.
