Red planaria flatworms. They feed on small foods like rotifers, phytoplankton and such, and will grow out of control unchecked. Creating a mat with their bodies, they can smother corals or parts of the substrate. If they get to the point of overpopulating the tank, there is the possibility that they may die suddenly all at once, and the toxin they release at death can cause further deaths and even wipe out a tank entirely. Starting today- Siphon out as many as you can and discard them in toilet.
best defense to rid of them is to add either a 6 lined wrasse, luare or even melanurus wrasse. You can also use salifert flatworm stop but of you do, be sure to siphon dead ones daily as they can release toxins into the tank. Check daily also for precence of eggs which are tiny and remove them