The flatworm exit itself will not hurt the anemones. However, the toxins released from the flatworms can crash your entire tank. Since your population is getting big, I would concentrate on siphoning them. You can siphone the water through a filter sock, putting the same water back in the tank, repeating as needed. This is time consuming, but the safest IMO. Once you get the population down you can use the flatworm exit. Be prepared for water changes, and run carbon after.