Both tags are used, but it seems to me they should be synonymous. The natural-lang-processing tag wiki states:
Natural language processing studies the algorithmic analysis and production of texts in human languages. It is closely related to computational linguistics. Modern NLP techniques make heavy use of machine-learning.
But, since there is a tag machine-learning, I think we do not really need this duplication, except as synonyms, since people tend to use one name or the other.