All weapon and armor items are tagged “Cannot be disenchanted” regardless of color (i.e., green or higher). Is this because the items were once equipped? All items are >level 100.
Some items you purchase from vendors (e.g. timewalking gear) or other sources cannot be disenchanted. If you did the Gear Update on that character it’s possible you can’t disenchant that gear.
You can normally disenchant gear regardless of whether it’s been equipped or not.