it is flatworms. can you take the rock out? if you can take it out and dip it in fresh RO water. Put a bowl of RO next to a bowl of saltwater from the tank. Dip it and shake it in the RO for 15 seconds and the put it in the saltwater bowl for a few minutes. Do this 3 times. Then put the rock back in the tank and do not put the rock on the bottom. Try to put the rock in a decent flow area where flatworms will not go. In the future if/when your flatworms get bad you will need to dose flatworm exit.