It is a little roundworm. A common example is a nematode.
Yes, most Platyhelminthes (flatworms) are motile. They move using cilia or muscle contractions, allowing them to glide over surfaces, swim in water, or burrow in substrate. However, some species are parasitic and may have reduced mobility or be immobile as adults.
Earthworms are in the Phyllum Annelida, the segmented worms. The earthworm body looks like a long string of checkers, arranged flat-side to flat-side. The roundworns, Nemahelminthes, are not segmented and are shaped like a knitting needle.