Looks like azalea lace bug damage. Only affects azaleas (and to a lesser extent rhododendrons, as they have their very own species of lace bug). They shouldn't affect flowering, although heavily infested plants like these do tend to be less thrifty. Azaleas in full sun and those subject to drought stress will be more susceptible.

Neem oil is a recommended treatment, however lace bugs are favored by a bunch of very beneficial predatory insects so I would resort to neem only under extreme duress.

Doesn't have to be neem oil but you do have to get the horticultural oil of your choosing to the underside of the leaves, where these pests are, or don't waste your time.

Commercial insecticidal soaps can also be helpful in controlling adults and nymphs on the underside of the leaves.

My best suggestion is to use a horticultural oil three or four times a year....early fall, late winter, and maybe early summer after the flowering period is over. I wouldn't spray a soap or oil on buds that have started to show color. Read and follow the directions carefully to avoid causing problems

Oils smother the nymphs and eggs, allowing you to keep ahead of the population. These insects may have numerous generations per year, depending on where you are, and can cause a deteriorating situation in just a few months.

Do evaluate the conditions. Unhappy azaleas are definitely more prone to lacebug infestations, as gardengal says.