Saw this in my mom's fish tank yesterday, there are 2 that I have found, she thought it was a glofish egg, def was worm of some sort, just not sure which kind.
Hi - I removed the video link because it was marked as private on youtube. Can you make it public and relink it? The forum will automagically embed it if you just copy/paste the url to the youtube video with no link markup around the url. Just paste in the url by itself inside your post.
It doesn't have a triangular head, so that's a rhabdocoela. No harm at all.