sounds like Flatworms.
Depending on your level of infestation, a six line wrasse could be beneficial, or if its a really bad infestation, Flatworm Exit made by Salifert. http://www.marinedepot.com/Salifert_Fla ... FE-vi.html
I've never used it but from other threads I have read, it works well - a key thing to do is remove as many of them as possible, BEFORE using FWE, and do a water change soon after so the toxins produced by the dying flatworms dont do further damage to your tank.
this guy has a good strategy on removal/eradication: http://www.melevsreef.com/flatworms.html