This is what I would have expected and it can be explained easily: The chlorophyll of the green algae contain nitrogen and for the
biosynthesis of chlorophylls they need iron because heme and siroheme are precursors. These are the two elements where you can limit green algal growth most easily.
Carotenoids which are the accessory pigments of brown macro and micro algae including zooxanthellae neither need iron for their biosynthesis nor do they contain nitrogen. This may be an evolutionary adaptation to a habitat with low concentrations of both.
I think a safe approach is the "natural concentration approach".
One or several (colimitation) nutrients are always limiting (maybe not in 100 % but in general). If the supply of nitrogen and phosphate is sufficient, i. e. by feeding, rapid growth, especially of SPS, will incorporate so much transition metals and other trace elements that are scarce in saltwater anyway that at some point they will start to limit coral and coralline algal growth. At the latest now macronutrient concentrations of N and P will start to build up.
If you relieve the "trace elements limitation" (sometimes not clear which trace element exactly but most likely an essential transition metal or iodine) by dosing trace elements the coral and coralline algal growth will recover and will consume more macronutrients, lowering macronutrient concentrations in this way.
This is
Liebig's law of the minimum.