If it is green then it sounds like Liverwort. You are keeping the surface of the compost too wet. Post a pic if you like.

Welcome, Lorna!
If it's liverwort, it's very shallow rooted, so it should just peel right up. Normally not much of a problem, except for small seedlings, though a thick enough cover could interfere with watering.

yep, sounds like liverwort, easy to remove by simply dragging it off with your fingers or small implement. Replace the top couple of inches with fresh compost & or gravel & keep slightly dryer. I remember working in a plant nursery years ago & the job I was given on my first day was to remove liverwort from a few hundred potted shrubs, have to say I enjoyed the task.