Wow that guy has some spare time
Interesting to know whats in there. I would imagine all sorts of stuff as its nutrients being skimmed out of the water, plus it usually sits there for at least a week or more, breeding (I'm pretty bad about emptying out my skimmer cup sometimes). Occasionally I find a couple brittle stars in the muck but as they have to rise up with the bubbles they're usually dead as they're out of water for too long. I've always wondered how they make it past the pump impeller.