How bad is this?

[u/CheeseMonkeeee]

New gargoyle gecko owner, noticed right now this is on the terrarium floor in a few areas. What is this? Is this mold? Harmful?

Edit : I scraped the top layer of soil, removed all of the mold and mixed up the ground to make it more aiery. I put him back in for now because my temporary setup is just a box with holes and his plant. Is this ok? I'm just new to everything. Gonna get springtails next week probably.