I've had flat worms since early on and they don't cause any problems. they come and go in quantity, sometimes the population explodes somewhat and they can look a bit unslightly, but I keep them under control by just siphoning them out each week during my water change. Another form of nutrient export .
I considered nuking them but did a bit of research on the net and found the likes of Bob Fenner on WWM saying much the same as above, just let them be, siphon them off, and learn to live with them.
Pies may have a differnt view, and he may even decide to come on the board a give it to you