What is the material leaking out of my plumbing drain stack?
Yes, it's a drain/sewage pipe. What comes out of a sewage pipe??? Sewage!
Get some rubber gloves and a dust respirator and clean that pipe and plug, a wire brush would be a good start. Then remove the plug and clean the threads on both surfaces. Apply some pipe dope to the threads and screw the plug back on to the pipe. Tighten as necessary to stop the leak.
That plug looks like it might be PVC so if the threads are destroyed, you might have to replace it.