Something is up. If the tank is 5.5 months old. When you compare the picture of your tank to what you see is anything off?
I ask because your Clams seem mighty colorless, as well as your leathers appearing faded. What appears to be elegance in the front right corner also appears to be shrinking and faded. When you got your livestock if it looked worse than it does now you need to find a new place for livestock as your buying animals that were probably half way to death.
Since you say things are growing and have improved then your doing the right thing. It's possible your plate is reacting to chemical warfare being released by another coral. Have you tried using carbon? If you haven't, then try for an observed improvement, remove it when you are satisfied with the results.
How are you treating your water for water changes? (R.O.? DI? get it straight from the backyard well?) Have you changed salt mixes?
With lights out how long does the frogspawn sweeper tentacles get, is it posssible they contact the plate coral?
You said you reduce the use of your halides. How old are the the lights and how does the tank look with them on?
These are a few of my thoughts based on the information, and photo's you have provided.
Erick