even if you dont feed anything it will still find something to eat if its anything like the crab i had. it sat in a rock by itself for 6 months in my 10 gallon, just him and the rock. and he didnt die till i forced him out with a straightened clothes hanger.
Hopefully you will be able to get whatever it is out soon. another trap you could try is take a like water bottle, and cut the top off about 2 inches or so from the top, and cut the top off to make the opening bigger if necessary. Then place the top inside the bottom to make like a funnel going in, and rough the plastic up a little so it can climb in, but once its in, it wont be getting back out until you pull it out.
If i didnt explain it well, check out this link...
Catching Your Fish - DIY Fish Trap
i caught one of my 2 crabs with this, the other with a glass jar like yote said