Nevermind I figured it out. If you are in this situation, right-click the worksheet that uses it and click 'Unhide All Sheets' there. It's basically like the worksheet takes on Dashboard characteristics.
This is hacky, but what I've done is added the tooltip sheet into the dashboard without a title showing or formatted text/shading as white and sized the sheet down to as small you can. Then I'm able to hide the worksheet.
Hope this helps!
The problem wasn't trying to find a way to make Tableau let me hide it. It was trying to find a way to make Tableau show it to me again.
Got it, well glad you were able to find it!