The problem is that leeches will also attack your shrimp. There was a thread concerning leeches a couple of weeks ago along with a picture of the leech. If you do a search you might be able to find it.
I found it this morning and managed to get a picture of it. It moves EXACTLY like a snail, very smooth. When I posted this, it was moving across my driftwood, which is very rough and bumpy. It must have been bunching up in the grooves of the wood. On the rock though, it just glided along. Here is a picture I took with a *hopefully* enhanced shot as well.
I am at a loss TBH-- I have never seen a worm like that (though I havent seen many worms in person honestly)
if it is a leech... you DO NOT want it in your tank... not a good option for snail control as it will destroy everything else as well.. and they can also carry disease