Have a look at the pumpkin question below...

Be better to have a photograph really - powdery mildew is the usual cause of 'white mould' but that is very, very unusual on hydrangea. What isn't uncommon on these is Hydrangea scale - that looks like white mouldy deposits usually on tips of stems, occasionally on the underside of leaves. Link below for Hydrangea Scale

https://www.rhs.org.uk/advice/profile?pid=557

If it is powdery mildew, the problem will shortly disappear with the approach of winter, since hydrangeas are deciduous. You can trim off the worst affected leaves if you want, and make sure you collect up the fallen leaves this autumn and clear them away.

Sounds like an environmental situation. Do you have it planted in deep shade? Does it get enough air circulation? Hydrandeas need open air. Consider relocating it to a new open location - bright dappled light, open air movement. They can take a few hours of sunlight. Just keep it consistently hydrated.