The discoloured leaves would indicate that it has a virus. In which case, there is nothing that you can do for it, sorry. It should be removed as soon as possible and burned or sent to the Council rubbish dump. Wait for other opinions, though, before following this advice.

It would seem that Hydrangeas do have problems with viruses and I would agree with Bulbaholic and you need to destroy. If it was a chemical deficiency, I believe it would it have a more regular discolouration of the leaves.

Definitely a virus. I would dig it up, double bag it, and send it to the landfill. Then leave the spot empty, especially of Hydrangeas, for at least a year, to allow the viruses to die.