yeah, I too had a hard time trying to find gray mold. Normally there is a gray mold that develops on the blooms and there is powdery mildew (it is usually grayish) that develops on the leaves. Is the mold on leaves or stems or ??? I did see some crown galls, plus stem damage and wilting of leaves that could be caused by nematodes but it was hard to tell clearly. Not sure if nematodes are a hydrangea issue where you live...

It could be a mix of the two. Frost breaks the cells and makes the leaves look dark green and then the leaves brown out and die. Pesticide damage can cause curling and other than spraying some water to dilute the chemicals, there is not much to do. The plant will generate new leaves eventually but it may take a month or so. Feel free to cut the leaves that look bad. Just cut the little string that connects the leaf to the stem in order to minimize impact on flower buds. Damage to these may be obvious once they start to open (or not).

There are some pests that feed off roots so ck for that. You could check the roots for root rot but that usually causes the leaves to be wilted 24/7, even after you water. Since you did not mention that the leaves were wilting, I do not expect that to be a problem but feel free to check to see if they appear rotted. To get root rot, you would need to be watering often, watering a lot and-or have soil that does not drain well, that is, you typically need a soil that stays wet for very long periods of time. If you have sandy soil, you could water daily and not have root rot because the water would drain very well and the soil would not stay wet for long periods of time. An example of watering amounts normal in the Summer would be to water 1.5 gallons of water per shrub when the soil feels almost dry or dry to a depth of 4".

I agree. The yellow to white ish leaves indicate iron chlorosis. If you live in the Piney Woods Region, it could be magnesium deficiency. If you are located in the Hill Country or the DFW Metroplex, your soil is likely too alkaline and it is needing regular amendments like aluminum sulfate (keep away from azaleas and rhododendrons) or garden sulfur. Iron chelated liquids can also be applied but you have to reapply them more often. See the package for directions on how much and how often to apply. You can get those products are plant nurseries. The granular products have to be reapplied less often than the liquids but the liquids work faster than the granular products. "Faster" is relative term as they all are slooow to fix the leaves... Whatever you use, apply the product and forget about it until it is time to reapply. If you keep looking for the leaves to turn dark green overnight, you will definitely fall asleep! Do not forget to reapply or else the soil will return to its original soil pH value. The leaf spots are cercospora, usually caused by overhead watering at the plant nursery or in the garden. If you use a sprinkler, set it to water the hydrangea at sunrise so the leaves are not kept wet for a long time. Other things you can are pick up plant debris, do not overwater to minimize high humidity and see if there is a way to improve air flow. https://ugaurbanag.com/cercospora-leaf-spot-on-hydrangea/ Down here, the leaf lesions become visible usually about a month after the Summer Solstice. I replaced my sprinkler with a drip irrigation. You can help minimize the leaf spot greatly by not getting the leaves wet using drip irrigation or soaker hoses. Discard the worst looking leaves. When the plant goes dormant in December-ish, discard dried out leaves and bloom in the trash. The are some cheap fungicides available but all they do is control cercospora, not cure it so I rarely buy them ($8). If you decide to apply it, start around the end of June per label directions.