"Ex" is not incorrect. Sometimes it is better to say "former." For example, "Bob is a former athlete." You can also use it as an adverb. "Sue was formerly the president of that bank." It depends on the tone. "Ex" has more negative connotations, e.g. "ex-wife." If someone is still in their profession, there is no prefix. You would just refer to them by that title, i.e. "Bob is a banker," or "Bob is in banking." The second is a more formal, and stuffier way to say the same thing. Source: Native English speaker. Professional ESL teacher (not on iTalki).