Soil pH:
Hydrangeas prefer acidic soil but they also tolerate alkaline soil past 7.0 to a point. But, as the alkalinity increases, the leaves change, except for the leaf veins. For example: the leaf may turn from dark green to light green, from light green to bronze or yellow, from bronze to yellow and finally from yellow to white. While making these leaf color changes, the leaf veins always stay dark green. If conditions do not improve, the white leaf with dark green veins will then die. If the leaf veins are not dark green -they are not in the above three pictures- then soil pH is the cause.
Sunlight:
New leaves that do not get sufficient sunlight will not produce enough or very little green chlorophyll. As time passes and as the leaf gets more sun, the leaf should turn bright green and then dark green. The leaf veins appear white too in this scenario that occurs more often in the Spring when hydrangeas leaf out. So there is no action typically needed on your part other than wait. If things do not improve, the leaves may drop on their own.
The same thing happens with other plants too. Roses are a good example. This time of the year, many roses are producing new foliage and, sometimes, the foliage looks a shade of red when the leaf does not produce enough green chlorophyll. With the passage of time, the leaves change to green. Roses are actually more sensitive than hydrangeas to this lack of sunlight and may develop new leaves with the red coloring in mid summer if the days are cloudy enough.
Hydrangeas that break dormancy early and that are being winter protected with leaves/mulch/blankets may also show white-ish leaves when the leaves unfurl while still winter protected.
Nutrient problems in the soil:
Some nutrient deficiencies can result of leaves turning white but this is rare. I would expect that if this were to be the case, multiple plant types in the same planting location might be affected in the same way or in similar ways. I would expect sandy soil to produce this issue more than regular garden soils. A formal soil test would be needed to confirm these deficiencies as the issue tends to occur with micronutrients. Those simple test kits sold at plant nurseries may not detect a problem if they only check nitrogen levels or the three macro-nutrients only (nitrogen, phosphorus, potassium).
Bottom Line:
Looks like a sunlight issue. I would not worry about those new leaves in the above three pictures. Note that leaves that are deeply inside the bush may tend to do this as they leaves above the white leaves cast shade and do not let much sunlight underneath. If the leaves above do not let much sun inside, you can consider cutting off the leaves or removing some branches/stems. To remove leaves on a hydrangea that blooms on old wood and whose stem/branch may now have Spring 2021 flower buds, I would recommend cutting the petiole string of these leaves (the string that connects the leaf to the stem/branch) so as not to accidentally cut off any Spring 2021 flower buds.
Watering:
Try setting the sprinkler to water near sunrise so infected foliage does not stay wet for long periods of time.
Fungus:
Cercospora leaf spot becomes more visible after the Summer Solstice. Apply the fungicide per label instructions to the leaves. Use clean sanitation practices too.
