unless you have a nutrient poor system,nuissance algae will continue to invade the skeleton of your cap before tissue overgrowth will happen.If your growing HA,nutrients need to be reduced.In nutrient poor systems,tissue overgrowth will occur in 2-5 days.That said,i would get your nutrient level down via WC,aggressive skimming,active carbon,etc.. then try fragging the dead skeleton off,if conditions are favorable,tissue will quickly heal over the broken edges.Sometimes with large M.caps,it might not be possible to just break off the dead area without damaging the colony in which case i would remove the coral from tank,scrub all algae off the dead part,and then use a superglue bandaid to prevent algae from invading