[moved]
spieszak nailed it. Flat worms. They can reach plague proportions, but not always. Nutrient control helps keep the population down. If they reach nuisance proportions, siphon as many as you can, nuke the rest with Flatworm Exit (from Salifert), followed with large water changes and a large load of fresh carbon to remove the toxins released by the death of the flatworms.