they are just an eyesore and they can reach plague proportions in some systems
They are easy to kill, the problem is the toxins they release when they die. If you can only see a few of them, then there is no better time than the present to nip it in the bud. Use saliferts flatworm exit according to directions up to a double dosage to be sure.
Get in a habit of dipping all new arrivals so you dont reintrocuce them or more importantly...something worse