Answer: If you've fed with a diluted liquid fertilizer, it's possible that some fertilizer splashed up on the leaves and what you're seeing is fertilizer residue. You should be able to rinse it off without harm to your fern. If you don't think this might be the cause, inspect the affected leaves for signs of mealybugs (soft cottony-looking masses), or for powdery mildew. Mealybugs can be controlled by dabbing them with a cotton swab dipped in rubbing alcohol. Powdery mildew can spread quickly, so it would be best to remove the affected leaves from the plant. Neither pest generally attacks ferns, but it's still a possibility!
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