Your photo is not clear, even when I zoom the computer to 400%. Even though I can't see clearly, I would assume they are aphids. As to the problem, it really does not matter what insect are, as the treatment is the same. In the future to avoid the insects and leaf damage, you need to spray the new growth flush a couple times, 5 to 10 days apart, starting shortly after all new growth has begun, with a good horticultural oil. You could also spray with a soapy water solution, or even neem. Personally, I never use neem, but it would work. Insects are highly attracted to the new tender growth of each flush and will attack them vigorously . Spraying with the above mentioned sprays will kill the insects, but will not undue the leaf damage. However, if you keep the tree insect free, all new growth will develop fine, The average life span of a citrus leaf is 18 - 20 months. Then the damage leaves will begin to die and fall off and new growth will replace them. In the future spraying new growth will avoid this from happening again. The best to you and this tree.

Control your ants and you will avoid a plethora of problems... aphids, scale, etc.

bdobush, that's one of the worst cases of aphids I've ever seen! It's not a disease, but a terrible infestation of insects. The white flecks are their cast off exoskeletons.

Yes, you can save the plant but you'll have to work at it a little. First, get in there with your hands and a fairly strong spray of plain water. You can dislodge a majority of the pests with the water and your fingers. Look for them on every surface of your plant, especially on the underside of the leaves.

Then, I would graduate to a commercial insecticidal soap. Better than home made potions with dish detergent, which can burn foliage. Just follow the directions on the bottle.

Neem is good to use as a preventative, an anti-feedant, and an insect growth regulator.

Once you go to the effort of cleaning your plant up, DON'T let it get that bad again! Keep after it with the spray from a hose and some squishing.

Good luck to you.

Thank you @Silica @John Merrifield @rhizo_1 (North AL) zone 7!