What is on your sand bed appears to be cyanobacteria. What you photographed on your montipora are montipora-eating nudibranches. This is a bad news. Please check all your montiporas in the dark with a bright torch and report back. It is because of these pests that I can never buy Montipora colonies from anyone, unless they are tiny frags, so that I can brush them as well as dipping them.