chiton? I think so? I'll snap some pics tonight, but, I have TON of them on the rocks. They usually come out at night and are hard to get off the rock.The recession is a bit strange tbh with the defined white area and no color fading above. It’s not also like AEFW or bug damage, so that’s why I wonder if bacterial. I also noticed this critter. Is that a chiton? Lastly, I’ve noticed irritation of my acros with digitate hydroids. Other than that, I’m as perplexed as you. Maybe it is indeed shading.