Cymothoa exigua, or the tongue-eating louse, is a parasitic isopod that severs the blood vessels in a fish's tongue, causing the tongue to fall off. It then attaches itself to the remaining stub of tongue and the parasite itself effectively serves as the fish's new "tongue"
Pretty sure I caught a fish with one of these once, was absolutely disgusted to find it as I was removing the hook.
Was it moving?
No it appeared dead