After using the boat on a very short trip last night (about 30 minutes of running on plane with very little idling), I found this this morning. It smells like engine exhaust (i.e., carbon). I'm thinking last night's running helped push this out the vent hole.
I've been idling this engine a lot due to using the boat in a river that is all "No Wake." Last weekend, we did a three day trip on the St. Johns that was primarily just going slow so the wife could take photos.
Of the last 25 hours on this engine, I'd venture so say that at least 23 of those have been under 2500 RPM.
I also had to use some Wally-world 2-stroke oil in a pinch last weekend (about a quart)--previously I had been using Penzoil Marine and Quiksilver.
The lower unit lube is clean and fine. My diagnosis for this is carbon buildup in the engine's exhaust. I'm planning on doing the Seafoam treatment.