The existence of this field in the Test Case must be a customization in your QC project as it does not appear in my v11 ALM QC system, which is using all default settings.
Under Tools > Customize, select Project Entities. Then expand the tree item for Test. This should show you two su-items > System Fields and User Fields. Expand these lists. In one of these lists you will find "Project" (or whatever the display name is for the field you are seeing).
If you then click on that, it will show you the definition of the field. If it is tied to a list, it should show you the list it is tied to. If you click on the Go To List button, it should open up the list for editing.
(Opinions and information contained in this post are wholly my own and do not reflect the opinions of my employer.)