That is an algae growing on old plant leaves. This type of algae will only grow on unhealthy portions of a plant. Just because it is sending out new growth, does not mean it is entirely happy. Decaying plant material releases sugars / compounds / ammonia that are needed for algae spores to germinate.
I will need some more information on your setup.
What is the distance from the light to the substrate?
What fertilizers are you dosing?
What plants are in the tank?
What other compounds are you dosing?
How many / how large are your water changes?
How often are you cleaning out your filters?
Is there lots of ambient room light or direct sunlight hitting the tank?
What are your tank parameters? Too many people say their parameters are "good" but in reality they are misled. I'm not accusing you of being misled, but giving actual quantified values will be more helpful than saying they are "perfect".
Anything else you can measure.
12-14 hours is too long, unless the light it very dim. 8-10 hours is plenty, even for a low light tank.
Manual removal is key. Remove all bad looking plant material, it will not recover.
Clean filters (in old tank water during a water change) every week or two.
Remove as much organic material as possible.
Get a good fertilizer routine setup. Healthy plants will inhibit algae growth.
Ensure your levels are within good parameters.
Start dosing a glutaraldehyde product such as Seachem Flourish Excel or Metricide 14.
Increase both the size and frequency of water changes.
Buy a clean up crew. Nerite snails, oto cats and amano shrimp are a good start.
Reduce lighting to 8 hours a day.
Reduce any sunlight that may be hitting the tank.
These are all a good starting point. If you can provide answers to the above questions I can help you out some more.
I hope this helps.