It looks similar but not quite the same, I'm pretty sure it's nothing harmful but it was weird how he was around the tiger nerite. One of the features of the worm is it can extend to 6-7 cm and make itself as short as 2 later. I also see them in the substrate most of the time. He tried to enter the nerite's shell but failed in doing so.Is it anything like this? Like an earthworm almost in looks, like it got lost lol? If so it's probably basically the equivelant of earth worm just the aquatic type.
I see what looks to be some lovely plants, if any of them came in a pot instead of being cultured it may of hitchhiked in that way and your new tankmate might of came from a dirty tank and it just wants waste to feed on if there is not enough built up in the substrate yet. They can range in colors from pink to brown from types and diets but they mostly look like earth worms. Tubificid Worms (Tubifex Worms) is a good article on them and alike things. Does any of that sound/look like it might be the case? If not i know there is parasites but few I know that big like horsehair worm which its probably not that but still look that up if this does not look like it.
Its hard to define its shape in the pic though for all I know yours is flat.