This is either a flatworm or a benthic ctenophore. Both have similar bodies and some species of both groups may be found on corals or soft corals. Benthic ctenophores have elongate thin ciliated tentacles they stretch out for feeding. We cannot see those tentacles here, but they can be retracted so may not at this time be visible. The ID as a flatworm is therefore tentative.
Created 7 March 2020