Take a sniff of it when you get home. Real green algae won't have much odor...
cyano will definitely have a strong odor.
I see problems in this tank that could easily cause multiple types of algae blooms, including
cyano.
You have 3 watts over this tank...that pretty much necessitates
CO2 injection, and frequent fertilization. When you have light, plants will use nutrients much faster, hence why your nitrate is only 5ppm. Plants also require enough carbon (in the form of carbon dioxide) to thrive as well. Once you hit 2watts per gallon,
CO2 should be considered. Over 2.5watts, and you're treading on thin ice without
CO2.
Soft water is the bane of
CO2 injection...without a Kh of at least 3 degrees (3 drops to change the color in the sample) you're running the risk of pH fluctations that could stress the fish and open them up to illness (less hardy fish may simply die). Calcium carbonate, or baking soda can be used to increase hardness to 3dKh. Flourish Excel is a carbon supplement, so you could continue using it per the label instructions.
Low nitrate levels tend to bring on
cyano, and it can be difficult to get rid of, sometimes requiring the use of anti-biotics (as
cyano is actually a bacteria, not an algae). With higher light, you need to maintain nitrates at about 10ppm, and phosphates at no more than 1.0ppm. Every other day you should try to dose trace nutrients, including some iron. To keep it simple and because your tank isn't huge, I'd use Flourish comprehensive plant supplement, and Flourish Iron for your trace dosing days. Start with 1mL of each, 3x a week, and increase until you find a threshhold where no more will help the plants.
Test your phosphate, but don't bother with iron. Unless you have a LaMotte Iron test kit you won't get an accurate reading. Cheap iron test kits just can't accurately measure the low levels we need in planted tanks.
Ok, I"m sure that's more than enough for you to digest right now
Good luck, and let me know how I can be of further assistance.