A picture of the type is needed, really, but it sounds like brown diatoms to me. This a pretty interesting type of algae as it lives inside a shell it makes from silicates in the water. Generally speaking, it is associated with immature set ups, possibly due to the presence of NH4 (including levels our test kits can`t detect).
Tank in direct sunlight might not be helping the the algae issue. Especially if you have your lighting on later in the day as well.
Light is a major driver for algae growth in the aquarium. You may well need to address the issue of sunlight falling directly on to your tank.
With regards to ferts i get a little confused. If you are not using ferts then there may be some nutrients that the plants need missing, but the algae can continue to grow, and if you are over ferting (which you are not) this may provide algae nutrition too. Perhaps someone with more nouse here could expand for me.
Algae can grow in RO water and it can grow in heavily planted, high growth tanks. Water column nutrient levels will not cause algae, but they will feed it. I have dosed an algae suppressed planted tank way beyond the nutrient requirements of the plants and not induced algae. If I was to stop adding the nutrients, the health of the plants will eventually suffer, causing them to leach ammonia in to the water column, which is an algae trigger.
Fortunately, brown diatoms are just about the easiest type of algae to remove, be it through elbow grease or buying a few Otos, which are great for this job. This type of algae has also been known to disappear over time as the tank become more mature and stable.
It has been attributed to low light conditions, but I seem to be able to produce it in 3.4WPG.
Dave.