I too thought it was Cyano, but usually it isn't that stuck to leaves and glass. I still have no idea what it is, but I fully agree with dukydaf.
I would cut the photo period to 6 hours. If you have a light that has a dimming option, make use of it. The tank doesn't need to completely dark, but too much light isn't helping the situation.
You need to increase your water changes to get the excess organics out of the water and I would change your filter media. Purigen is awesome and works, but you need some type of fine filter material to catch the stuff that is floating around the tank. You didn't mention how often you clean out your filter, but it should be once a month as even though the filter takes stuff out of the tank, all that gunk is still there. If you don't clean out your filter, this is contributing to all the excess organics in the water.
All the leaves that are covered should be removed. If it hasn't spread to your substrate, it will if it is cyano. If you find any on the substrate, that too needs to by surface vacuuming. People freak out at the mere mention of vacuuming a planted tank, but there is no reason to. You won't be doing a deep vac, you are simply removing the excess stuff from the surface. Amano staff vac their tanks with no harm, so there is no reason nobody else can do the same and not hurt their plants.
There are blackout methods that supposedly help, however, if you don't get to the crux of the problem this will continue to be a thorn in your side as you won't address the cause of the problem with just using the blackout method alone.