Acoel Flatworms (Waminoa brickneri) on Fungia coral
Diver's lodge House Reef, Lembeh.
These worms can infect corals in large numbers (see also next spotting) and harm their host by shading and by destroying the protective mucus layer, reducing the coral defenses. So no good news to see many of these on a coral.
Waminoa brickneri is a worm from the Convolutidae family, which belongs to the Acoelomorpha strain. While the majority of acoels live in marine sediments, some, usually identified as Waminoa sp., have been found associated with corals, living closely appressed to their external surfaces.