hmm, i want to say theyre flatworms, only because ive had something similar to that a few months back. everyone told me not to treat with flatworm exit and they they would go away on their own--- they were right; theyre not in my tank anymore.

the only reason im not certain is because i never saw them swim.. they were always attached to something like my the stalk of the zoanthids, on chaeto, or just hang out on glass