workfellow vs fellow worker

workfellow and fellow worker both are nouns.

Word	Noun	Adjective	Verb	Adverb
workfellow	Yes	No	No	No
fellow worker	Yes	No	No	No

As nouns, workfellow and fellow worker are synonyms defined as:

Other synonyms of workfellow include co-worker, colleague.

workfellow (noun)	fellow worker (noun)
an associate that one works with	an associate that one works with