How can even a small amount of citrus "cook" the outside of seafood?

The "cook" in this case is not actually a cook. It is the acid in the citrus (all citrus contain... citric acid), which interacts with the proteins and precipitates/coagulates them, which is essentially the same process as happens when you heat protein.

In both cases the interaction causes loss of secondary structure in the protein, which results in essentially insoluble protein that then precipitates out of solution. Denatured proteins are generally whiteish ( e.g. egg white/albumen is ~10% protein, which is clear before heating, but white when denaturation then precipitation by cooking).