Normally I have tank lights on for 7-8 hours, this is pretty regular with lots of uk fish keepers who have planted tanks. 12 hours is a little too long, especially if you have strong or intense tank lights.
This looks like it’s filamentous or green thread algae.
It’s usually found in tanks with an imbalance of light and nutrients in the tank, actually quite common in newly set up tanks where new plants have not fully established.
If memory serves, lots of fast growing plants will compete against the algae for nutrients, as essjay mentioned earlier using an old toothbrush or something then twist and wind the algae around this and remove as much as you possibly can.
The fast growing plants and your manual removal of these green thread algae will eventually get rid of it, takes a while though, couple of weeks to 3 weeks perhaps if you remove as much as you can daily. This algae grows fairly quick so have to keep an eye on it until the plants and tank naturally returns the tanks balance back to where it should be.
Putting your lights on a daily plug timer will help as well, aim for 8 hours or maybe 9 if your lights are not particularly strong. Can get a cheap analogue plug timer from loads of places, Argos, Wilkos or any hardware store will likely have them as well as online of course.