Strips don't test for everything - they don't test for ammonia, for example - and nothing measures everything which builds up in an aquarium.
Most of us do weekly water changes of at least 50%.
Algae is caused by an imbalance between lighting and plant fertiliser. Too much or too little of one or other make it more difficult for plants to grow while algae can grow in conditions where plants grow slowly. So duration of lighting can cause algae if they are on too long.
Spectrum of lighting also has an impact. Plants need red and blue in the spectrum and some green also helps. Many lights have too much blue, and algae can use blue to grow. The spectrum is the K rating of the light. It's usually four numbers followed by K and may be written on the light itself, in the instruction manual, the packaging or the manufacturer's website. If you can find it, tell us the K number - and how long each day the lights are on for.
The slimey looking green algae - does it peel off in sheets? If it does it's more likely to be so-called blue green algae which is actually bacteria. This grows where there's too much organic matter in the tank - uneaten food, fish poop etc - which comes in with jaylach's question about vacuuming the substrate.