This is an undermurk. They are rarely seen detached from their host in this fashion. Typically they are to be found affixed to the undersides of the flippers, or webbed feet of marsh creatures such as the Fever Fen, numerous species of plasmatic monsters, and the Blue-bellied Bog Runner.
Hidden away in the membranous skin-folds of these swamp dwellers, undermurks can hardly be called idle passengers, for while they do indeed feed upon their host, the relationship is extremely beneficial. The multi-chambered shell of the undermurk acts as a sort of filtration system removing poisons and toxic enchantments from the bloodstream. A healthy swamp monster or marsh demon is, almost always, absolutely filthy with these little brutes.
A system of undermurk shells can be employed to assemble a rather crude device for purifying liquid. However, though oft attempted by marsh-trippers and Waste Survivalists, an undermurk purification system is persnickety business, to say the least. Few have succeeded, rather more than some have been envenomed, and most have died in terrible, terrible ways.
Undermurks will instantly sour a basin of milk, up to 2 feet away.