How did I come up with this theory? Allow me to explain.
About the only nuisance pest/algae I have not had is bryopsis in any of my tanks, but I have used fluconazole in other bryopsis overrun displays a few times. My LFS services many tanks so they have considerable experience with it as well. Most outcomes were quite good. A very small handful of very bad outcomes. That is covered in other threads, so I will leave that debate aside.
Vibrant I have dosed a small handful of times in my own display to help manage this or that, when my herbivore population has gone on strike, or I get some bubble algae. I never dose it higher than the suggested levels for a fairly "clean" tank. My experience with it is generally pretty good, but I am very cautious and patient with it.
Early Summer, I removed a bunch of my old rock from my display as it was overrun with encrusting SPS plus a bizarre vermetid outbreak. I built some nice structures with dead rock, made a modest effort to cure it, and chucked it in. Well, you know what happened then. Got GHA all over it within a couple months. CUC and herbivores weren't getting it done, half the tank looked terrible, so I started in with some Vibrant, and now maybe 5-6 weeks in with it.
-- All of the GHA that is exposed to my lights got weak and died out.
-- All of the GHA that is shaded, seems rather content.
If you follow the fluconazole thread, you will see that this is also how fluconazole behaves. The cellular disruption process (as described by Jose Mayo) requires active photosynthesis to occur. Any bryopsis (or GHA) that is shaded remains in tact. I am aware that Vibrant is marketed as a "blend of bacteria". Given that it does tend to lower nutrients gradually over time suggests to me that it does have some bacteria in it.
WHAT DO YOU THINK ABOUT MY THEORY?
To me, this is a smoking gun. Like many/most(?) of you in the SPS forum, I love me my acropora and the risk -- however small -- of a bad outcome for my expensive tenuis is just not worth it. I will just have to let this algae phase pass on its own.
Two pics. The first is my (now) pristine rock that is fully exposed. The second pic is my happy GHA, hidden under an overhang.