Well, I searched the site and found several references to flatworms and various similar creatures. Many folks seem to think that they are actually an indication of a healthy tank. Sure freaked me out at first.
Interesting thing is, I don't ever see them in the tank proper, substrate or on the glass. I guess that the tank inhabitants (cherry red shrimp, various denominations of snail, cherry barbs, panda cory) are feeding on them. There are no predators in the filter though.