Well, if it is caused by water pooling, or evaporating selectively from certain areas, I would think that raising the low spots would help, by adding a little soil mix or scratching it around to level it. That MIGHT help either fungus or salts, by spreading the bad effect around evenly.
Increased air flow might help, if it is surface dampness causing mold or fungus.
>> we shower the plants every day with water.
If it is salts, maybe using bottled water, rain water or distilled water would prevent it from getting worse. Certainly "less fertilizer" will reduce salt loading.
>> this problem is only appearing in two specific areas
Just out of curiosity, what if you propped up the terrarium a few millimeters or 1/8 to 1/4 inch on one edge? That would make the opposite edge damper than the raised edge. If the low edge gets worse and the raised edge gets better, you would at least know that they want less water.
If the whitish spots stay right where they are, maybe it IS the plant or the soil right there that makes trouble.
I forget
