Smell it. If it's active slime flux will have a strong fermenting odor.
My first guess was some species of phytophthora but it's purely a guess. Is this on the same property where you suspect phytoplasma? Cuz it sure sounds like you have something in the ground there.
above the ooze is a hole. gently scrape it out and look for signs of insect activity. a hand lens can really help here.
the primary purpose of writing the attached was to highlight the differences between infestation and infection. check codit's summary at the end. probly shoulda used charts/tables to clarify this.
methinks you got a bug there good buddy. does not look like flux or any other kind of infection to me based on pics.