I have a birdnest that was looking great, I had a problem with an algae bloom and I raised the mag to fight it. my birdsnest bleached almost completely with a day. I broke off the bleached parts did a water change and stabilized everything as quickly as I could. Since then the birdsnest has come back and is doing great. Your coral looks the same as mine did. I have plenty of crabs and hermits that always are climbing and picking at them. None have caused any damage. I'd say you have a swing in water parameters. what salt are you using?