Thanks guys. Here is some more info / photos.
The algae kind of gets on everything. Glass, plants, substrate. it starts kinda fluffy and gets thicker and darker and becomes a layer of slime.
Tank has been going about 6 weeks, but I used my old tank's filter media.
I'm feeding lightly every 2 or 3 days (only residents are a few neons).
Lighting is pretty intense, on for 8 hours a day. I think it is 4 x 9w cool white LED lamps over a 7 gallon tank. 4.9wpg. I suspect this is my problem.
EDIT: I only have test kits for CO2, ammonia and nitrate/nitrites. What are other good test kits?