The tank needs a powerhead, point it at the water surface to agitate the water and help oxygenate the water. I see your clown at the surface which indicates low oxygen, and algae consumes oxygen at night and that much algae will cause low oxygen issue's in the tank. Flow is very important in a saltwater tank.
Algae also loves low flow and stagnant area's, so a powerhead will help against that too.
Don't overfeed, feed small amounts several times a day. If you see food in the tank 30 seconds after feeding, you fed too much so feed less next time. Rotting food creates higher levels of dissolved organics in the water which feeds algae.
Clean up crew will help with small amounts but can't do anything against bad infestation, so you have to do your part to keep it in control.
If you have no corals, you can reduce or turn off the lights for a while.