a lady on youtube that has been growing indoors for over 20 years told me that if that is happening on top of your soil its a sign of too much moisture.
she says its not bad theres nothing wrong just stop watering it so often and it will go away.
Fungi growth ( saprophytic ) can be from excess moisture - i would get it when i blended mushroom compost in my soil blends. Not particularly harmful to plant as this fungi feeds on decaying matter.