I've had a problem with two of my coral plugs on the top shelf of the aquarium recently so I removed it and the next rock below it to re-glue the plugs. I put the top rock in the fish acclimator while I got the 2nd rock out of the tank. After the new glue and replacement of the rocks, I found this in the acclimator.
I have not added any rock to this tank since I set it up (December) and all of the rock came from the frag system I had in the basement. (i.e. - no new rock for over a year) All corals were on plugs so I don't think he would have come in via corals (but who knows.) The only shrimp in the tank are an old (3-4yrs) pistol shrimp, a purple lobster (about 3"), and two new cleaner shrimp (one week).
The hitchhiker is about 1-1.25" in length, translucent body with the black stripes, and very thin - a little more than the ink "cartridge" from a traditional ballpoint pen.
My inclination is to release him back into the tank since he's been there for 'quite awhile' anyway and I have seen no harm - but, any reason why I shouldn't???