I also thought "salt accumulation" when I read the text, but it did not look that bad in the photo.
Unless those are very salt-tolerant plants, their health suggests that it is NOT excess salt crystallizing on the surface!
But possibly there is some very insoluble mineral in the soil blocks or water that precipitates on the surface without high concentrations of sodium and chloride. (Do you use tap water? Do you have "hard water" with Calcium and Magnesium salts? If that was the problem, watering with rainwater or distilled water might help ... but how do you get salt out of a terrarium that has no drain?)
I could easily believe mold or fungus. It's humid and organic and the air sounds still. If it is mold or fungus, improved air circulation and reduced humidity might help. Take the lid off and run a fan? The remaining water will evaporate rapidly and make things look worse if it is salt, but maybe look better if it is mold or fungus.
