I am a bit unsure on exactly what you are questioning, so I will add extra data.
Yes, I was researching macro-algae, but what I consider long term nuisance algae is usually macro algae.
Basically, hair and bubble algae being the most annoying and stubborn problems.
I deliberately grow hair algae in an ATS.
I do want to emphasize a very specific experiment I have been doing the last couple of weeks.
I replaced one of the old mostly-red LED modules with the new ones I am building for the main tank that are mostly blue.
This was to get a really feeling about how nuisance algae would do. Plus a few other things, but irrelevant here.
The algae under that light is definitely not growing as thick, and is browning a bit.
Does not prove anything about coral of course.
Some of it is just basic physics. I cannot remember the exact sources on algae absorption.
All I could find with a quick search was this. Mostly on general water red-absorption, but does mention and even
show how chlorophyll content changes water color.
ocean blue
I think you are really underestimating the amount of algae in the water, even apparently clear water.
But regardless, the water itself absorbs a ton of red.
Sure, lots of different corals do different things, as does algae. More of a general statement.
But I find it fairly amusing that two pictures you posted had
RED coral.
If they are red, that means it is being reflected, not absorbed, and thus not used as much.
(Similar to Coraline algae BTW)