Answer: The mushrooms are not harming the plant. Mushrooms are the fruiting bodies of fungus that is feeding on decaying organic matter in the potting mix. You can scrape them off if you wish. Eventually, when the organic matter is decomposed, the mushrooms will stop on their own.
Cordyline does best in very bright light. Allow the top inch or two of soil to dry out between thorough waterings. This plant is sensitive to fluoride in the water, so you might try watering it with rain water or fluoride-free bottled water. It is also sensitive to excess fertilizer build up in the soil. Fertilize only lightly with a water soluble fertilizer such as 10-10-10 plus minors. Read and follow the label directions for the weakest rate and apply monthly during the growing season of spring to fall.
Based on your description, your plant could be suffering from overwatering, excessive fluoride or fertilizer, or a fungal or bacterial infection. If you think you have been following good care recommendations, you might consult with your local county extension to see if they can determine the cause of the problem more specifically. Good luck with your plant!
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