Don't want to write this by hand? GodCraft sets minecraft:is_hidden_when_invisible for you with a switch.Get GodCraftOverview
minecraft:is_hidden_when_invisible is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Lets the entity go unnoticed by hostile mobs while it has the Invisibility effect.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:is_hidden_when_invisible": {}
}