Yep algae. It's a microscopic spore bearing plant, everywhere and unavoidable. But algae growing on the top of soil is not harmful; it will not penetrate deeper, it will only grow on the surface where it is exposed to light. Put some kind of light blocking cover over the top of the soil and don't worry about the algae anymore.
If you have uncovered nutes exposed to light (as RIXUK hinted) they will breed algae. Algae breeding in your nutrients robs your plants of nutrients (algae is a plant). When it dies it'll decompose and potentially feed plant eating micro-organisms.