Consider extraneous, "Not belonging to, or dependent upon, a thing; without or beyond a thing; foreign", and synonyms like superfluous ("in excess of what is required or sufficient") or pleonastic ("Using an excessive number of words"). Other synonyms of superfluous include excessive, extra, supernumerary, surplus, unnecessary, extravagant, some of which apply.
Answer from James Waldby - jwpat7 on Stack ExchangeConsider extraneous, "Not belonging to, or dependent upon, a thing; without or beyond a thing; foreign", and synonyms like superfluous ("in excess of what is required or sufficient") or pleonastic ("Using an excessive number of words"). Other synonyms of superfluous include excessive, extra, supernumerary, surplus, unnecessary, extravagant, some of which apply.
My preference in this instance would be to use either prejudicial or detrimental as they convey the sense of 'harmful' as requested.