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Idk. I wish I could help. Could be parasites or gill flukes, could be bacterial or viral. Is it just swollen? Is his fin missing? ( Looks like it's missing from the picture) Is there any white stuff by that area? Is it reddened by that area? Has his behavior changed?
People who do have more experience than me, who can give you a good answer, will want to know the size of your tank, the inhabitants of the tank and all your water parameters.
BTW. Is that a water spot or does your angel fish have a swollen gill, too?