by centaura » Aug 28, 2012 3:26 pm
Sounds like an algae problem, I second Pauline's recommendation that you need to scrape away the surface soil in that area and add a thin layer of a dryer mix. Algae is hard to avoid if it is present in your water supply, just be aware of the tree's staying too wet and scrape it off if it becomes a build-up again. Skipping a day in watering to dry the algae off is also another solution (Since it dries out fairly quickly, just watch that your tree doesn't dry out). If there's enough extra moisture in your soil to keep algae going, then it probably wouldn't hurt your tree to skip a day of watering and dry out a bit.
-Cat