Why do i get green dusty algae on the glass of my fishtank?

Between water changes i slowly get a buildup of a green dust like algae on the glass and the pebbles. It rubs off easily with a cloth, but i can never get rid of it out of the water. After a week or so it slowly comes back. I do two weekly changes of water, and all is balanced ok, although i can’t reduce the nitrate any lower than 40 due to tap water levels. Lighting is on for eight hours/day and water is well aerated, with external power filter.

Thought i’d beat it over time, but have failed.

I’m UK based 🙂