target all players that have an item in their offhand with custom_data

the item would be this minecraft:amethyst_shard[minecraft:custom_data={ice_gem:1b}] but how do I use it in a statement like this execute at @s run effect give @e[nbt=!{Inventory:[{Slot:-106b,id:"minecraft:amethyst_shard"}]}] minecraft:slowness 1 3 true