Could very well be those terpenes that were mentioned. I really wouldn’t know much as I’ve never owned a leather coralThis is exactly what happened to 2 torches and 1 hammer (all my LPS at the time). In a span of 1 week, they all just dissolved away. I've had them for months before that. There were not added at the same time and the hammer was not from the same source as the torches. It made me hate my tank tbh because no one could figure out what the heck happened. Since then I've added some cuprisorb and started doing 10% water changes every 2 weeks. Now i have 2 duncans, 1 hammer and 1 dying candy cane. The duncans and hammer haven't have been in for a long time so I dont want to get excited. I just hope they make it this time...
PS: The rest of my corals are softies like zoas, shrooms, cloves, gsp and 4 leathers. What if the leathers have been releasing toxins all this time? I do run carbon and change it out monthly.