One week ago I dimmed the lights and the GSP started to act normal again during the week, the zoanthus and macroalgaes didn't show any reaction to the low light during those days.
Three days ago, my GSP had one cloudy eye that healed by itself.
Today I connected one more bulb and the GSP is still acting normal. Now I'm really happy, because I noticed him hunting for food among rocks and caulerpas! That was so cute.
I thank you all very much!
PS: That saltwater tank has an old skimmer and an algae scrubber that keeps my nitrates undetectable by test kits, I use tap water to mix the salt and there's no diatoms/hair algae in the main tank. I still have the bag of biorings used in the brackish tank where the GSP lived in the beginning, those biorings keep my biological filtration ever since. As you can see, I planned a cheap saltwater tank for my GSP with macroalgae and a few inverts, and those filtration methods are keeping a good water quality for them.