Organic matter often is saturated with natural oils. When submerged, oil can be released, to float to the surface. Right now, don't worry about it. If you don't have much organic matter other than the trees, keep watching and see what happens. If the condition persists or has a strong oily odor to it, post about it again. It could be something different.

Teach a man to grow fish...
He can teach to catch fish...