Here's why it's not a part of speech on its own:
* Prefixes are added to the beginning of a word to modify its meaning. They don't stand alone as words.
* Parts of speech describe the function of words in a sentence (e.g., noun, verb, adjective).
Examples of words with the prefix "mal":
* malfunction (noun)
* maladjusted (adjective)
* malpractice (noun)
* maltreat (verb)