Answer: If it is indeed a fungus growing on the top of the soil, you're overwatering your plants, making perfect conditions for root rot and other diseases. On the other hand, the white coating may simply be a mineral build-up from either your water or your fertilizing practices. It's hard for me to determine without actually seeing the problem. I think it's time to repot in fresh potting soil. This will remove either problem and stop it progression. After repotting, water your plants thoroughly and then wait until the top inch of soil is dry before watering thoroughly again. All of your tropical plants will do best in bright, diffused light such as you'd find a few feet from an east facing window. They should be happy in average household temperatures, but tropical plants don't like cold drafts so keep your plants away from any air conditioning vents.
Best wishes with your indoor garden!
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