odd behavior with 'using'

I created a class in one folder and then moved it to another in my project.  When i go to reference it with later files it resolves in the old location instead of the new?  Do I need to run something to have a reference updated?

for example:

public property1 column1 { get; set; }

property one was in models.folder1 but I moved it (drag and drop) to folder 2

in the cs file I have:
using project.models.folder2

but its grey even though property1 was moved there but if I change it to:
using project.models.folder1

it changes to white and the error goes away?

I've tried to do a build, rebuild and clean but none of this allows me to update the reference.