Small reddish-rust colored creatures may already be in your tank. Or you may buy a coral and they'll appear as hitchhikers later. They may appear on the glass/acrylic, on the substrate, or on live rock. Due to their extremely thin bodies, they are called flatworms correctly. They appear almost two-dimensional, barely a flap of skin. They can move through the tank, elongating the front part of their body like a sail filling with wind, latch on to the nearby surface and pull the rest of its body forward.