Hi Andy,
The only causes I can think of are the usual suspects: Uprooting the substrate without doing a water change afterwards, excessively high fish loads and NH4. If you're innocent of the first two then try to figure where the third might have come from (dead fish unaccounted for, unplugged filter causing bacteria die off, over-cleaning of filter media etc. etc. etc.)
Theoretically, unfit or non-growing leaves have a higher probability of leaching NH4 and nutrients back into the water and are the most likely attach points for the algae. Slower growing plants are the next most likely targets.
Micronutrients? You didn't mention the dosing but presumably you dose if you're basing your scheme on James'.
Cheers,