Just today I noticed a bright green mold/fungus under the substrate in my terrarium (I can't insert a photo for some reason, but It appears branch-like in shape). I have cypress mulch which I keep pretty damp, however in the year that I have had the terrarium this has never been an issue. I will probably change the substrate, however I am unsure whether or not this could pose a threat to my gecko (african fat tail) and if I should get a cleaning crew to avoid this in the future.