That is a green water algae bloom and reducing light won't help and if left untreated will eventually deplete all the oxygen from the water no matter what you do. Leaving the light on is just speeding that process down!
With a bloom that bad you need to use a UV sterilizer to kill it off which in this case might take a few days. Yours is very bad! This is the unit many on the forum have used with success...
Green Killing Machine Internal UV Sterilizer with Power Head at PETCO.
I can't stress this enough.... turn the lights off and leave them off!!!
Cover the tank, every inch of it to block all light from getting in, top, sides, and bottom. No light, no looking, not even for a quick peek. Leave the tank for 3 days. Your fish will be fine. Do this while waiting for your UV sterilizer which I strongly suggest you get even with this black out. I honestly haven't seen an algae bloom this bad in a long time.
After the 3rd day black out do a HUGE water change, like 75%. All that dying algae is going to cause a big ammonia spike which will hurt the fish.
Once it's gone stop running the light that long. No more than 8 hours, period. Put the light on a timer so it can only run that long. This is not good for the fish and how can you enjoy a tank full of green water.