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Unfortunately, your photos did not come through. I am unable to say, for certain, what it is. There are a few causes, though.

Fungi and salt buildup would the first thing suspected. Many fungi develop a relationship with plant roots, and are beneficial. If growth seems normal, and it isn't stressed outside of the weather, then this is not likely a cause for concern.

If this is salt buildup, then flushing with quite a bit of water can help. To remove salt buildup from potted plant soil, rinse the plant's roots thoroughly under running water. Next, create a salt removal solution by mixing equal parts water and distilled white vinegar. Soak the roots in the solution for 15-20 minutes to dissolve any salt crystals. After soaking, rinse the roots again to remove any remaining salt. Finally, repot the plant in fresh well-draining soil and maintain proper care to prevent future salt buildup.

This search page will take you to several articles on the care of your Cannas.

https://www.gardeningknowhow.com/search?searchTerm=canna