To refer to someone currently in a job, you don't need a prefix. You can say, "She is a nurse," etc. Or if you want to emphasize that this is their current job, you could say, "She is currently working as a nurse," "He is working as an accountant."

Thanks Kevin for your explanation. It really helps

"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).