Hard to tell in that picture, is it actually yellow toner? Almost looks like it could be an oil stain. Changed the colour drum for yellow?
We get the same thing in our beater C6000. They usually dump out some of the developer. The tech said there is too much and it spills over the meter blade. Almost always yellow too. I don't know if he is full of crap or not, but when he is finished it is gone.