Found more pooh today in the same general area as the last one. I'll post a pic later. This same activity happened in the 60g hex tank. The pooh would always appear in the exact same place in the tank. Each morning when I would check the tank I could count one new piece of pooh added to the pile. Today's is at least an inch long. I am baffled. I have asked people at the LFS', but everyone thinks I'm nuts.
If you really want to find out what it is then here is an invasive technique for you to try. This works well for mantis removal too.
Get some 1/4" or 1/2" nylon mesh screening and make a simple wood frame and staple the mesh to it. Pull the rocks out of the tank where you think the critter is and place the rocks on the mesh screening. Underneath the mesh place a tupperware container full of saltwater. Mist the rocks regularly with saltwater and a spray bottle to keep the bacteria and coraline alive. Eventually whatever critters are in the rocks should eventually seek the water and fall through the mesh into the container below. This could be a long process taking a day or two. Just an idea. The problem is that it requires you to remove the rocks from your tank which can be a bit invasive. Just throwing out a suggestion.
Well, today I found another dead hermit, so I wonder if these events are related. I have been occasionally finding hermit claws laying in the tank but always thought they were from a pair of hermits fighting it out for a shell.
Are you sure it is not a molt? Sometimes it can be difficult to tell the difference. Are you noticing less and less hermits in your tank? If so the may be killing each other. The solution to this is just to provide extra shells of varying sizes. Or maybe the mystery critter is a mantis who is snacking on your hermits?