i had a massive infestation of flatworms in 2 separate tanks at one time. i introduced a pod eating fish( in this case it was a whipfin fairy wrasse) and within a month or so, the flatworms died off. the wrasse didn't eat them, he ate the pods that they eat.
they starved off.
check this link out- http://macropyga.googlepages.com/
this guy gave me the idea to add something that competes with the flatworms (ambush predators that eat pods) for food.
i'm not sure how this method will work in your seahorse tank though. i assume he'll be competing with the horses as well.