What's your water parameter like? Do you have effective and matured filtration?
Start with a water change and add some water conditioner. Then follow with some aquarium salt and Melafix. Make sure the water is well aerated.
What's does the white spot look like? Is it a protrusion or an open wound? Are there any other affected areas? We need more details to try to diagnose what's wrong.
Cheers,
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