I wonder if the top of the right fork is leaking fluid? It would make sense as the bike leans to the left so it would run down to the stem nut as there is some dampness there. Make sure the brake line isn't leaking anywhere above it as well. Try to get some of the fluid below the stem nut on your finger and smell it (Yeah, this does sound crazy and I'm not joking). If it has any kind of petroleum smell it's probably fork fluid.