What is this and how can I get it to stop leaking?

Thanks for all the advice everyone I did try multiple time opening and closing and letting it shut on it own to no avail. Water was cold when I first opened it and water heater has been off all day. I will drain and replace. That's the only way I can see to make sure it doesn't fail on me in the near future. Going to take it off and head over to Home Depot. Thanks for the advice. I love forums. And yes I should have bought a DP... Someday......