Red Planaria, flat worms. They are not the "bad kind." They're only harmful under a couple of different circumstances.
#1. If they reproduce to the extent that they cover your corals, blocking out light. They actually can reproduce to this extent.
#2. If you treat them, without first removing as many as you can. When you treat them, with Salifert Flatworm Exit, they become toxic, when they die. It's important to remove as many as you can prior to treatment, AND to remove as many dead ones as you can as soon as they start to die, after treatment.
Typically, a 3X dose of Salifert FWE is safe. In other words, use 3X the recommended dosage.
Other options are certain Wrasses, which are known to eat them. It does look like you've got quite a population already, and they're already decent sized.