ok, let me go over GSP for you, When it doesnt like its location it closes up, when it doesnt like the water flow it closes up, When it doesnt like the paramaters it closes up. IME with gsp and Iv had it in many tanks from large reefs, Fish only displays, and pico reefs. I would say that it does best in intense lighting, high flow, and stable salinity. your lighting is not enough, I know it says online that they dont need much light but honestly this is just not true in most cases. I have one group of just that does alright in the darker area of the tank but it doesnt grow much and its just not that happy. alot of the time if you look at pictures of GSP thats doing really well its growing on rocks or on the glass at the top of the tank. at my LFS
they have a coral troff tank that has a return thats right at the top of the water and the GSP coverd it because its about 2" under the water with a large MH
over it. however to your point about the detris you got off it that can cause the GSP not to open if its been closed awhile but its not the reason it closed in the first place. I would try giving it more direct light and water flow before upgrading your lighting. move it and dont touch it for 2 days. if its not open in 2 days then try another spot. I would try a few spots before upgrading your light unless you want to anyways. but if it doesnt get enough light it will start dying off if it stays closed to long.