Green on top of soil

All my pots and modules have gone green on the top of the soil. What causes this?
I am trying hard to balance the watering of everything and lift the pots and trays to see if they are light but I am having the odd pot or odd module that is dry  so I then add more water. Am I over watering

If you can do it without damaging the plants, try 'tickling' the surface of the soil with a suitable implement (e.g. cutlery fork or plant label) to disturb it a little (the algae don't like it although it probably won't cure it altogether.)

You may have overwatered, but over time it tends to happen anyway at this time of year when humidity levels are often quite high, and in my experience is more unsightly than a real threat to the plants unless it gets really out of control.

Forgot to say I always water in the tray not in the pots which is why I am a tad confused as to why the soil is green. Should I do anything like scrape it off or anything?

Whilst it might be a symptom of overwatering, I tend to agree with JayG, its one of those things that happens and is more cosmetic that a real problem.

I usually just scrape it off or break it up with a plant label.