Occasionally fungi and slime molds appear on the surface of growing media in containers. These organisms will not hurt the plants or roots. They are involved in the decay of organic matter and are more likely to occur when the growing media remains wet for prolonged periods of time. These fungi should not be confused with algae which is a problem. Algae creates an ideal environment for fungus gnats and shoreflies and creates a barrier on the surface making it difficult for water to penetrate.
Growers that are concerned about appearance can scrape off the fungi, and some growers have been known to carefully vacuum it off the surface.