Because the Wall application is a group it will be transparent to everyone. This means that everyone within the group regardless of which project they are a part of will be able to see one another.
If you feel that you do not want certain users to be able to see each other you can remedy this by creating a new group and putting those people in it.
As seen below both Brad Segura and Lucy Letman, while in separate projects, are still displayed on the wall.
If you look at the picture below you will see that only Lucy Letman is displayed on the Group Wall. This is because Lucy is the only one that is part of a project within this group.
When making a post on the wall you can make it private. To do so click on the link below the comment box that says Everyone. From here you can click on the checkbox next to the names that you want to share this post with.