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What alkalinity test kit do you use?
I've had the same problem with too low of nutrients even while feeding a bit heavier than you at 4+ cubes of frozen per day + phyto and reef chili on my 165. My cheato recently died back pretty good due to Nitrate hitting 0.00 (I also noticed Iodine hit zero). Unfortunately the feeding more method just didn't work for my tank so I had to start nitrate dosing. Nearly immediately after starting to dose nitrate and iodine my cheato is coming back strong and dark green again. Corals also look much better with nitrate dosing. I also turned my skimmer pump down to mid speed setting and skim very dry which seems to help increase nutrients but still not enough.
Absent nitrate dosing I think it's pretty normal for Cheato to go through growth /die-off cycles as nutrients fluctuate (or rather it it grows to the point that it strips all nutrients causing a new cycle).
Perhaps trimming it more often might help? I'm experimenting with this now. I also noticed that it can become quite dense and compacted if left untrimmed which over time really increases the amount of cheato in a given space (mine literally became a cube that would hold it's shape when removed).
I need more information on this thing called nitrate dosing.