Looks just like my osteo DinosIts presented like dinos, a little thick to be diatoms. A microscope, even a kids one will confirm.
24 hr blackouts have never effected any of my corals including acros. Just make sure whatever you use to cover the tank is not letting any light through. It needs to be complete darkness to trick the osteos into the water column. Then UV will sterilize them if you have one.I have a bunch of acros won't this negatively affect them?