Red Planaria to my understanding are photosynthetic, so they just cover coral in search of a good spot to lay out and get some rays. (Corals are usually in higher light areas, so the red planaria often lay on top of the corals or around them to take advantage of the light - nothing to do with the coral itself.) As far as I'm aware, they don't actually eat any dead coral tissue. If you're having an issue with tissue loss on your corals, it could be from the toxins red planaria release as they die or from other issues.