US (MA): Molds on the surface of growing media
To control these molds, reduce irrigation and increase airflow to lower the relative humidity in the greenhouse. Growers that are concerned about appearance can scrape off the fungi. Loosening the compressed media will also help to inhibit the fungal development by drying it out.
These mold fungi should not be confused with algae (green slime), which are a more significant problem. Algae creates an ideal environment for fungus gnats and shore flies. It is a food source for fungus gnats and shore flies. Fungus gnats not only feed on algae and other fungal growth in the growing medium, but also on plant roots, and can transfer plant pathogens through their mouthparts. Algae can form an impermeable layer on the media surface making it difficult for water to penetrate into the growing media.
Source: UMass Amherst (Geoffrey Njue)