A couple thoughts...
First, what is the stability on the alkalinity? A spike or fall could do that.
You may want to check PAR to make sure it isn't too high. Also make sure flow in that area is low.
Other things to consider are an iodine dip and possibly Cipro (in tank or in QT). This would help if they have a bacteria problem.
Do you have any angelfish or other fish, or even shrimp/crabs that may be picking at it? Likely at night for inverts.
Hammers, torches, etc. can be finicky. And it can be hard to figure out the exact problem...
Hopefully this gives you some ideas to look for!