Hi
@Dennisjeffrey and welcome to the forum.
Chances are it is algae, not moss. A green layer on top of soil is often caused by algae, which thrives in warm, wet, bright environments. Algae are notorious for taking up residence inside of greenhouses.
Algae form a thin layer on top of soil and grow in warm, moist, bright conditions. Moss, on the other hand, forms a thick mat (often called a carpet) on soil and grows in moist, dark conditions.
Algae will not work directly to kill your plants, since it is not a parasite or a disease to your plants. However, algae can indirectly harm or kill your plants in a couple of ways. First of all, algae can compete with your plants for moisture and nutrients in the soil. Over time, algae can form a hard crust on the soil, which makes it difficult for water to penetrate to the soil and to a plant's roots. Second, if algae form a thick layer on your soil, it can retain too much moisture, leading to mold or disease for your plants.
Since algae thrives in moist conditions, the best course of action to get rid of algae is to dry it out a bit. Water in the morning, not the evening. When you water too late in the day, the soil stays wet overnight. If you are watering from the top, consider watering from the bottom instead. This allows the surface of the soil to dry out, preventing algae growth.
If you are using grow lights, do not have them on more than 16 hours a day. If your greenhouse is very bright all day, use some shade cloth to tone down the light a bit.
Are you using potting mix or garden soil to grow your plants? Potting mix is best as you will not bring in algae spores as you might using regular garden soil, but there is also a downside to potting mix! Peat moss, vermiculite, etc. tend to hold moisture better than garden soil. Allow the surface to completely dry out before watering. If anything, we tend to over water our plants.
Use a fan to keep the air circulating and help dry the surface of the soil.
Hope this helps!
