Cyanobacter bacteria loves nutrients, low oxygen levels, slow water movement, and red light. Cleaning the tank can usually help but you need to do big (75%) water changes and gravel clean the substrate every day for a week or more. You should also wipe down the glass with a disposable paper towel or something you don't want anymore. You need to physically remove as much of it as possible.
Wash ornaments that are contaminated too.
Increasing aeration and water movement around the bottom of the tank can help.
If you have old (more than 12 months old) fluorescent lights, relace the globes and starters. Use globes with a 6500K rating (K
is for Kelvin). If you have LED
lights they are fine and you don't need to touch them.
Reduce the dry food going into the tank.
This stuff also works and pretty quickly.