About algae and fungi on potting media of houseplants
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Algae are single to multicelled aquatic plants that can grow under moist conditions and form a greenish layer on the surface of the growing media.
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Fungi are usually multicelled organisms that will also grow on the growing media surfaces under moist conditions and form a white to grayish layer on growing media surfaces. Both of these organisms can be growing together at the same time or separately on media surfaces. Their growth isn't usually detrimental to plants.
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However, if growth is excessive, a thick mat of algae or fungi can form on the growing media surface. This mat can become dry and crusty on the surface, and interfere with water absorption, and infiltration into the growing media.
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In addition, the algal or fungal mat can serve as a food source, and attract other pests such as fungus gnats, and shore flies. The larvae of these insects feed on plant roots causing wilting and stunting of plant growth. The adult insects can also pick up and spread disease spores from plant to plant by their movements between pots.
Management
To prevent algae or fungal growth on the growing media surface allow the surface to dry between waterings and keep the humidity low by ensuring good air circulation between pots. If a mat forms on the pot surface, either break it up with a small cultivation tool, such as a fork or remove and discard the upper layer of growing media along with the mat of algae or fungi.