"This is my coworker."
Translation:To mój kolega z pracy.
Yes, it is. Polish word "kolega" has a broader meaning that English collegue or co-worker. If you use "kolega" without the addition "z pracy" it could mean any kind of acquaintance ranging from your co-worker, that you barely know personally to your friend. There is a quite funny tendency among some Polish native speakers to actually use "kolega/koleżanka" instead of "przyjaciel/przyjaciółka" (friend) with no meaning change. Some use it interchangeably .
It sounds as if you used the word "to" as a noun, which doesn't really make sense and frankly, sounds offensive, as if you considered your coworker an "it".
Instrumental can be used in an "X is Y" sentence (where X is either a noun phrase or a personal pronoun: Marek jest moim kolegą/On jest moim kolegą), not in a "This is Y" sentence. In "This is Y", "Y" takes Nominative.