Looks like a regurgitated pinky to my limited experience. I say this in part because I don't see anything like urates, suggesting which end of the snake it came from. The pink part looks like skin, maybe that is the head toward the bottom of the picture, and the other end is the most digested. Out of curiosity, which end on the mouse went down first? Also smell: did it smell more like bird poop (poop) or rotten eggs/rancid meat (regurge).
Just wanted to point out, poop doesn't always break down in water. I've tried with both known and unknown samples, and sometimes the dark solids in the poop just don't change much, but they were very small and dense to begin with.
It would be helpful to everyone on this forum to add this example to the poop/regurge thread:
http://www.cornsnakes.com/forums/showthread.php?t=97648&highlight=poop+thread
For sure follow the Kathy's regurge advice and use a pro-biotic. I'm using Benebac for reptiles (it was faster to get a hold of than Nutribac) and as far as I see, it's the same benefit, very similar bacterial cultures, only no vitamins (does Nutribac have vitamins? I forget). Take it easy on handling and slowly work the little guy back up to normal prey size. I did this for my regurge baby and he's doing just fine now.
Almost forgot to mention, is he going into shed soon? It's important to determine the cause of a regurge as far as possible to correct any husbandry conditions that might be off, or to know whether this is a snake that absolutely should not feed in blue, or whether the possibility of something internal going on is raised. If you can't find anything wrong and no shed soon, and there's another regurge, it might be time for a vet trip.
Good luck to you and your scaley.