If a fish is injured, perhaps a scrape from a rock, there is a clean scoop of flesh missing from the site. This heals just fine when the water parameters are really good. Perhaps a few extra water changes. This would reduce the population of (possibly) infectious bacteria.
There are several diseases or parasites that can look like a white spot. Pictures are too blurred to see.
1) Ich. I do not think this is it. Spot looks too big unless it is several Ich organisms really close to each other.
2) Columnaris, a Gram negative bacteria. More often on the main body of the fish, but it sure can attack any part. Look up some pictures of Flavobacterium columnare, Flex, Saddle Disease (it has many names)
3) True fungus. More often seen on an injured area, or may move in on some tissue that has been compromised by Columnaris or other bacterial disease. A fungus usually look like white strands, perhaps wooly.
4) Lymphocystis, a sort of wart-like growth.