I'm a big believer in "relax, have a homebrew," so I try to not overreact when something unexpected happens, but this gave me pause.
When I opened the lid of my primary to rack the beer, I noticed what looked like flakes/clumps of off-color (black) krausen floating on top of the beer. There's definitely some typical tan-colored krausen stuck on the sides of the fermenter, and this stuff is in stark contrast to it - it's very black. See attached pics. I of course tasted the beer and there are no off-flavors that I can detect.
Details: this was a BIAB batch of a red rye ale that fermented for 2 weeks at 68 degrees F (FG = 1.013 - higher than I'd like, but ok). The yeast was Nottingham. This is the second time I've brewed this batch, and the only thing I changed was substituting a couple pounds of 2-row for a couple pounds of wheat malt and rice hulls. Needless to say, I didn't see any of this last time. I didn't notice any of these flakes (or other black/burnt material) coming out of my brew kettle. I use hop pellets in muslin bags during the boil, which were of course removed. I didn't dry hop or add anything that wasn't wort to the primary.
I'll see how it looks after a couple of more weeks of aging at room temp, and I'll reassess then.