The cape Shadow Prowler’s Cloak, off General Drakkisath from UBRS isn’t listed when I am looking at optimizing my gear.
As far as we know, that cloak has not been implemented yet and will become available with phase 5 (i.e. Ahn’Qiraj). If you’re only looking at available items (max phase 4) that cloak will not be shown.
If you already own that item, please add a snapshot id by clicking the green “Help” link above Best in Bags to create a support post so the AMR team can change that.