Collect 3 Toxic Horror Droplets for Donova Snowden in Winterspring.
I've studied the sample you returned with, and while I can't say for sure, I believe it contains traces of toxic elements. However, the only way to know for sure is to gather some as a comparison.
I believe that the Deadwood furbolg may be gathering these elements from other corrupted creatures in Felwood; in particular, the elementals living in the Irontree Woods, called toxic horrors.
Collect a few samples for me to compare, but be careful -- it's very dangerous there!