Blue green algae (Cyanobacter bacteria). It loves nutrients, low oxygen levels, slow moving water and red light.
Reduce the dry food going into the tank.
Do big daily water changes and gravel cleans.
Increase aeration and water movement.
If you have old fluorescent lights, replace the globes and get something with s 6500K rating. If you have LED lights, increase the blue so you have equal amounts of blue, red and green light.