Found this on the corner of my glass, it’s pretty big, and it moves! look like near it are some tiny docs that might look like eggs. In my tank i got nassarius, trochus, and strombus!
I would set a trap for them. Any container with some food inside will work. Just wait for them to crawl in and dispose of them. Remove the eggs as well. I dont really know much about flatworms as Ive never really had to deal with them