Yup. Sounds like planaria 'flatworms'. I've never had any luck keeping them gone, but Flatworm Xit will kill some off for a while. The last time I used it, though, I lost every monti and stylo in the tank. I just use natural eradication these days (wrasses, mandarins, etc.). I've got several tanks on one system. Some have them, others don't, so where my fishes keep them at bay (everywhere by the frag tank it seems...). They are toxic to fish when they die off en masse, so if you do treat for them, siphon as many out as possible and be prepared to run carbon and do a water change after treatment. You can knock thme off of incoming frags with a dip in CoralRx or Revive (Revive is a good bit cheaper and does just as good a job, IMO).