It's pretty simple :)
I assume your indexer parameter type is string
. Then in assertion instead of
x[TypeName]
use
x[Arg<string>.Is.Equal(TypeName)]
As result your assert should look:
manager.Site.AssertWasCalled(x => x[Arg<string>.Is.Equal(TypeName)] = Arg<IList<FcCacheObject>>.Matches(y => y.Count.Equals(1)));