So I’m actually running into the same issue and I’ve tried almost everything… I have a lepto that did the exact same thing, white on the ridges until the entire thing died and that coincided with birdsnest and stylo bleaching. Still haven’t identified the culprit in my system. I have so far ruled out normal parameters as the cause, I run a trident at 8.6, 440, and 1400. I also dose aminos nightly. My nutrients are 10ppm nitrate and 0.05 phosphate, right within a normal range. I don’t feed the corals often but I feed LRS reef frenzy daily which has lots of good stuff in it, so I don’t think feeding is the issue. I have 3 gyres in a 4 ft tank and I’ve reasonably ruled out flow as the problem. I have 2x Gen 5 XR30 blues running on 40%. I had people say it was too high light causing the bleaching, so I rented a PAR meter and found the PAR at those locations to be 140-150, so definitely not from over exposure to lighting. Under exposure to lighting is interesting, because the effects of that on coral shouldn’t be bleaching. I would understand lack of growth or coloration, but low lighting shouldn’t cause RTN or bleaching. I do have a peppermint shrimp but I’m skeptical that any of my livestock are causing this issue, this has happened to about 5 lepto frags and various other SPS over the past 6 months while I’ve tried to trial and error a way to a solution.
So where I am now…. I believe chloramines or other additives in my RO water could be the issue so I’m upgrading my BRS 4 stage to a 7 stage this week. I’m also in the process of ramping up my lights, although I’m skeptical that’s the cause I have nothing to lose trying it. After that I’m kinda out of ideas… unfortunately I’m grasping at straws, checking random things like the tubing I use for water changes or containers I use for my mixing station, cause all the usual stuff seems to be in check.
I know that didn’t help, but hopefully it helps you rule some stuff out too. If you want to bounce some ideas off me let me know, I’ve been down this path for a while now ha