Since your using RO
for your water have you changed the RO
membrane and/or filter cartrages? Over time they become less effective and should be changed every so often.
Very strange that your phosphates are testing 0 and nitrates are testing 0. Usually on a tank that old you have some nitrates and there is a small level of phosphate that is in natural sea water but excessive amounts can be the fuel for undesirable algea.
Going on the light possiblity. if this algae is somewhat a recent development how old are your bulbs. Over time the bulbs lose their specitrum and start to show more into the red and yellow end of the speciturm and this is also fuel for algaes.