New update.
This may sound like one of those random responses that suggest some wild ideas, but!
As I have previously documented, I have removed several frags to other tanks, and more recently brought one back into my tank after it had recovered.
At the same time a chance conversation brought up the subject of plug in air freshners and how toxic they can be to an aquatic environment. No I am not saying that this is conclusive proof by any means, but.
I had a plug in air freshener a foot away from the tank and 2 others in close proximity, I have consequently removed them all and guess what total transformation in the tank over the last week or so. The hystrix frag in question continues to have no blistering and now has multiple growth tips and all other corals have woken up and are beginning to grow again.
It may be worth checking if you have air fresheners in close proximity?
I know this is not conclusive and mostly anecdotal, but I've been keeping corals for over thirty years and the transformation is note worthy.
Hope this may help some people, will be interested to hear people's responses.
Kind regards justin.