There are times when misspelling a word is done purposely, like when writing dialogue. If the only option is to add the word to the dictionary to remove the highlight, words get added that, at another time, may not be misspelled on purpose, but you will not be notified if it is now in the dictionary.
Add an option to “ignore misspelled word” as opposed to adding them to dictionary. This would remove the highlight and ensure that if the word is misspelled some other time unintentionally, it will still be highlighted