This looks like RTN/STN (Rapid/Slow Tissue Narcosis).
Did you have any parameter swings in the last week or so?
It may also be AEFW (Acropora Eating Flatworm) bites, you can check this by looking with a magnifier to look for eggs at the exposed areas where tissue was and at the base, as well as to use a Turkey buster or a small pump to try and blow them off the tissue.