I wouldn't worry too much, although they're massive hehe! I've ran into similar tiny brown flatworms like this and I did nothing about it. Usually in new systems because there isn't much biodiversity you can get blooms much worse with these guys. I have seen the surface of course substrate entirely covered with these little brown flatworms. You can find them on the glass and basically everywhere. They will disappear in time you don't necessarily need to use some flatworm killer. Just keep doing your WC's and manually remove as many as you can. That way you don't get all the toxins and excess nutrients as they decay. My last option would be a Blue Velvet Nudibranch. Although getting one seems to be almost like hunting Bigfoot, searching for Atlantis or photographing a Unicorn.