That looks like spider mites or other small bugs. Some are too
small to really see without a magnifying glass.
When I see this on my citrus, I spray with malathion. I squirt
some liquid soap into the sprayer after adding the water; the soap
is a cheap wetting agent. Since malathion persists only about 3
days before decomposing, a repeat spraying might be needed in a
week or so.
No spray or other measure will undo the damage. Spraying will only
stop the damage from spreading and getting worse.
David E. Ross
Climate: California Mediterranean
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