The slimy looking stuff is not briopsis and not what I would call gha. I guess I would need to see the whole tank and the algae close up in the water to be sure. Under microscope will not help me much, but could help others if you have access to one. Since you are in school, you could ask the science teacher to use one and take pictures. They may love the idea.
Definitely cut any light source. You can even take black garbage bags around the tank to keep the light out. If you have any corals, just do this for 3 days. If no coral, do it for two weeks.
Looking for that FTS when you have a chance.