[Subsea]

Yes, not only have I seen that, I am dealing with some of that in my 25 yr old 75G Jaubert Plenum display tank. In my case, most of the organics on the macro is detritus from what was a 6” dsb that lost its detrivore crew due to a Malanarius Wrasse. However, it can also be some of the leakage from differrent macro algae. A large part of the complication is that DOC is a by product of photosynthesis. In our glass tanks, when DOC from macro lowers the redox of the water, this favors a reducing environment which is not what you want in your reef tank. Use GAC to remove DOC. As much as possible, promote good gas exchange
in as many places as possible. I use bio ball’s in the first chamber of my mud cryptic zone refugiums

Let’s assume you implement using GAC. There will be a time delay on the removal of DOC. DOC is continually being leaked into the water. Once the nutrient source is reduced you need to do a major clean up of your macro. I would pull macro out and do a major vacuum of substrate & rock. With two open buckets of water, clean macro in one bucket and put cleaned macro in second bucket. Inspect macro to see if further cleaning is necessary.