I assume you are referring to azalea lace bug (Stephanitis pyrioides), they leave dark fecal spots on the undersides of leaves. Lace bug is a common insect pest of azaleas and rhododendrons in this region. Drought-stressed plants are more susceptible to lace bugs, so make sure to give your azalea some water during the dry summer months. Next spring, you can spray the underside of the leaves with a strong stream of water to dislodge the wingless nymphs. Pesticides need to be applied in the spring (mid-April to late May), when the young nymphs appear. Most pesticides used for lace bugs work on contact, so you need to have good coverage, including on the undersides of the leaves, for them to be effective.

Insecticidal soap and horticultural oils can be used, they smother and kill the insects. Neem (azadirachtin), pyrethrins, and Spinosad can also be used. Avoid using acephate, bifenthrin, cyfluthrin or imidacloprid on blooming plants or if there is any possibility of pesticide drift onto blooming plants nearby. These pesticides are highly toxic to bees.

For more information on management, refer to: https://hortsense.cahnrs.wsu.edu/fact-sheet/azalea-azalea-lace-bug/