Should I worry about these white marks on my exterior wall?
Generally in housing, the white marks (efflorescence) is caused by salts in the cement washing out to the surface when wet, and being left behind when the water evaporates.

If there's any salt being added to the system (I did some research in college on a brick wall built with seawater exposure), then over time the efflorescence can be part of the system that will cause the cement to break down over time. 

However, if your patio is inland and you don't use any salt on it (like de-icers) then the efflorescence is cosmetic and fairly harmless. If it bothers you there are chemical solutions (bleaching agents) that you can find at the hardware store that can remove efflorescence. They will not prevent it from coming back, it will continue to effloresce until all of the salts are removed from the cement.