The nut looks like its cracked on the inlet side the right side of the valve not the top? Although I can't see the top? This will require the plumber to come and isolate or bleed the tank and replace it.
The fact that the left side of the pipe has a 90 going down wondering if this isn't a make shift pressure relief valve?!....
Take 2 more pics one on the top and one farther back to get a better view.