If after having a wet dream, upon waking up, one only finds traces of vaginal discharge and is pretty sure that it is neither mazi or mani, does ghusal become obligatory in this condition.
It seems that the vaginal discharge you are referring to are merely the natural lubricating fluids creating in a woman’s body when she is aroused. This, however, cannot be deemed an ejaculation of sorts. If you clean this area as you would after urination then that suffices. Hence, the discharge of this fluid falls in the same category of flatulence (or breaking wind) and urination.
I hope I have clarified the issue.
God knows best.