I would definitely treat them. Flatworms have a community-suicide trigger. Basically, if one feels that committing suicide is best, they'll all trigger themselves to die at the same time (but one or two will survive to plague your tank the next go-around.) The release of toxins is extremely deadly to an aquarium (depends on species though...would need to see a better pictures...but chances are, you have the bad ones.)
If you're on a tight budget, manually removing them is best. I think it was Melev that had a handy "flatworm funnel" he made. Basically, it's airline tubing attached to a filter sock. You can re-use the water after it filters through the sock. Just throw away the flatworms after siphoning them out. Keep doing that whenever you see them. It's a pain, but oh well if you're on a budget.