Weird gray sludge

So the other day, I racked a tripel I'd brewed about a month ago into a secondary fermenter. The primary I'd used was a 6.5 gallon plastic bucket. Around the rim of the bucket, where you'd expect the kräusen line to be, was this gray sludge with white veins in it, like nothing I've ever seen before. I took a sample of the beer and tasted it, it tasted a bit raw and boozy as was to be expected from a strong and young beer, but no off flavors or anything. Has anyone ever seen anything like this before? The only thing I did different with this beer than any other beer I've done was that I used some oats in the grist, but I can't imagine that that would have caused this gray sludge. I sanitized everything thoroughly as I always do, I open fermented it (with a BIAB bag cinched over the bucket to keep bugs and whatnot out) as I'd done before, and only sealed it when the kräusen receded. Any ideas?

I have actually noticed something similar in beers with a high percentage of oats, especially in combination with wheat. Additionally to your findings, when I harvested the yeast from the bottom of these batches and let it settle in a glass container, there was a clear layer of dark grey-brown material sitting on top. When looking at my brewday notes, I realized that all of the batches that happened to, were batches where the lautering process did not result in a clear wort. Therefore, I figured that the sludge came from inadequate grain bed filtering during lautering and that it was just grain particles.

Originally posted by derA
I have actually noticed something similar in beers with a high percentage of oats, especially in combination with wheat. Additionally to your findings, when I harvested the yeast from the bottom of these batches and let it settle in a glass container, there was a clear layer of dark grey-brown material sitting on top. When looking at my brewday notes, I realized that all of the batches that happened to, were batches where the lautering process did not result in a clear wort. Therefore, I figured that the sludge came from inadequate grain bed filtering during lautering and that it was just grain particles.