It could be a possible bacteria infection as a result of adding something like a large LPS trachyphyllia for example or maybe euphyllias, possibly a hammer or torch of some kind. Hammers and torches carry tons of bacterias that can cause a lot of damage to your reef. So much, that I personally would never add one to my display even if it was super aquacultured. If I had to guess, I would say that you have the shroom mixed with other corals that are infected but the others are somehow temporary immune. Some corals are more sensitive than others even ones within the same species. I've seen this happen to a lot of people and they all claim, more or less, the same situation, "everyone is fine except for this Bounce". If you have any euphyllias in the same tank with your Bounce then I would try separating them. You can also try cleaning the water gently by running a moderate amount of carbon if you're not already doing so.
I first thought shrimps could have been the reason why it's damaged but on second thought, if this was the case, you should be able to see some sort of damaged flesh or maybe the shroom shrinking all together but it doesn't look like that's happening.
