That looks like a NEMA 10-50 receptacle, which I would expect to have two hots and a neutral at 240v. If that is the case, you don't have a true ground, and ideally should figure out how to get one. You can run a new line with H-H-N-G from the main panel, run a separate ground from the main panel, or use the controversial 3 in - 4 out spa panel.
Before you do anything, you should verify that you have 6awg wiring for the 50a circuit, that it really is H-H-N, and that it is on a 50a breaker on your main panel.