What do you use for phosphates for the plants? For the light level you have, the plants aren't going to grow really fast, but they still need some phosphate. Also, once you get plant leaves infested with algae you almost always have to prune away those leaves before you can get control over the algae. You can kill the algae on the leaves, with a 1.5-2X overdose of Excel, but the leaves will mostly die anyway, which will lead to more algae later.

My 2.5 gallon nano uses Excel for carbon, no CO2, relatively low light, and the plants in it grow very slowly, but almost entirely algae free. I dose KNO3, KH2PO4, CSM+B traces, and Excel in it, every other day. It has a 27 watt PC desk lamp about 6 inches over the water. So, you should be able to make your tank grow algae-free.