Answer: Based on your description, it could be a fungal problem (soft) or it might be mineral deposits (crusty). In either case, gently scrape away the top layer of soil. Take care not to overwater the plant, especially since it is growing at a relatively cool temperature. Increasing air circulation could also help.
If it is crusty, periodically water the plant very thoroughly to the point of running through the drain hole, several times in succession, to flush out any mineral or salt buildup in the soil due to fertilizers. This would be done during the growing season when you are fertilizing and the plant is in active growth.
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