When a project is 100% completed you have to mark it closed for it to disappear off your dashboard.

To close the project from your workflow dashboard you will need to select the project and then close it, as seen below:

To view all your closed projects you can filter for "Closed" on your workflow dashboard, as seen below:

To close a project from inside the project, you will select "actions" > and then select "close project", as seen below: