If it's just on the the coco surface, it's not affecting the plant. The coco surface can collect all kind of plant debris, and with nutrients can probably support some fungal growth. But even if a fungus strain somehow is growing on the surface of your coco (presuming this is not a systemic root rot-type fungus infection), the plant is not affected. If neither roots nor leaves are showing signs of fungal growth or other problems, you don't have a problem.
But what about your humidity and ventilation? How damp or over-saturated is this coco? Unless you've recently watered, coco surface should be fairly dry after a few hours of light/heat and ventilation, such that no fungus could grow on it.