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This could be a number of things--too little or too much water, over fertilizing, leaf scorch (if planted in too much sun), etc. It may help to prune the plants to help rejuvenate it. This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/azalea/pruning-azaleas.htm Here is additional information that might help: https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm

It is most likely azalea gall, which will commonly produce stone-like growths that may be pale yellow, light green or brownish in color. At any rate, the leaves will need to be removed and destroyed (including any foliage on the ground around the shrubs), as this is caused by a fungus and will spread. You can also treat the shrubs with neem oil to help control the fungus. It will also help with any pest issues that may arise as well.

It sounds like you might have scale on your plants. This article tells more about them and their control: https://www.gardeningknowhow.com/plant-problems/pests/scale-bug-how-to-control-plant-scale.htm Whatever control you use, make sure to apply it once a week for 4 weeks to break the life cycle of the bugs.
Also, here's an article on other possible problems that might affect azalea: https://www.gardeningknowhow.com/ornamental/shrubs/azalea/common-azalea-problems.htm

This article will help with the azalea lace bugs and their treatment: https://www.gardeningknowhow.com/ornamental/shrubs/azalea/azalea-lace-bug.htm Also, I would suggest using neem oil. It is safe and effective for treating pests like these. Here is more info: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm To find out if your soil is acidic, you will need a soil test. Your local extension office can provide this for free or low cost, or you can purchase a soil testing kit.

This article will help with amending your soil: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/amending-sandy-soil.htm These shrubs prefer well-draining, acidic soil. You may want to test the soil pH. You can do this yourself with a soil testing kit or through your local extension office. Then you can treat as needed. This article will give suggestions on raising the soil acidity, if necessary: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/raise-acid-level-soil.htm

Is it a greenish blue insect scale or a greenish blue substance that looks like scales? If it is an insect, try neem oil. I have found that useful on scale. This article will help with that: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm If it is a substance, then it is likely lichen, which can be unsightly but does not kill plants. This article has more info: https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm