Feel your pain - I have been dealing with this issue in mass the last 8 months or so. Also suspected the lil white bugs. Mine act and resemble red bugs and are about the same size. To good of a chance they are copepods tho as I have found nothing of an sps pest like these. Looked for aefw - nothing. Checked water over n over - nothing - changed out a million possibilities - nothing. Yet still over night death.
I truely have been led to believe it is bacterial. The way the rest of the tank being fine - things checking out yet still having overnight wipe out on a piece.
Being an sps guy - this last year has been terrible in dealing with this. Seems to of always been an underlining in my system after the 1st year. And have dealt with it since.....3years later lol
Someone find a cure lol
Good luck with the issue - it is not an easy one