Field Formatter: how to know where "i'm"
A Field Formatter has no awareness of when it's used. It's simply just a unique way to display a field.

You want to use something like Entityreference ViewMode Selector. And create 2 formatters for the Field Collection. 1 formatter is the default output for collections, 1 formatter is the viewed as entityreference formatter.

When you set the entityreference, you would set the formatter as needed.