It is understood that the envelope contained a flower which had died and dispersed its seeds.
Earlier, the force said it was aware of the concern such a discovery may cause “after the events at the nearby Heaton Park synagogue earlier this month".
Two people died and three others were taken to hospital after Jihad Al-Shamie, 35, drove a car at members of the public before stabbing people outside the synagogue on 2 October.
Adrian Daulby, 53, and 66-year-old Melvin Cravitz were killed, with police later revealing Daulby was hit by police gunfire as officers tried to stop the attacker.