It is definetly algae.
chances are its a mix, high phosphates in the water, lack of water change problem with filter bacteria, is the filter flow working?
You should check the phospate levels of your water. I never heard about the free iron in the water but this makes sense, it might be worthwhile switching from iron based fertilisers to magnesium based ones as i have heard magnesium only benefits plants...
Check you timing of your lights, you could try intorducing aquraium mineral salt to help the plants intake of nutrients from the water. Introduce snails they are the only life form that you can introduce that feed on it- shrimp will not touch it.
What fish are you stocking in the tank, if you have high polluting fish they will add to problem, plecs for example produce so much waste.
My dad has it in his tank and it is at last retreating we have tried all of the above, apparently once its dead it stays in the tank, it takes along while to dissolve once dead, but you know when you are winning as it is easier to clean.
if yo have introduced any ornaments lately you might want to remove those, as new objects that have not been cleaned can introduce problems in to you tank.
I have also read that if you break up your day time hours then this will disrupt the algae growth, but not damage the plants- havent tried this with my dads tank yet but i might soon.
Let us know how you get on, its a bad problem and there is no simple answer i am afraid, just keep attacking it.