Aquariums: What is that worm

This came with a newly purchased pelia moss. It is almost transparent, and only its guts (??) are visible. There are three black dots on its head, but I cannot see if it is arrow-shaped. It moves by stretching its head and then curling up. When it curls up, it is shaped like a drop of water 💧. The visible structures are only at the rear of the body.