We have examined your citrus leaves and it appears to be a bad infestation of mites. I would spray over and under the leaves with an insecticidal soap spray; you may need to repeat every 7-10 day to prevent further infestation from occurring.

If you could first wash the entire tree with a water spray in a bath, or outdoors when the weather is warm above 55 degrees, that would help. Mites like warm, dusty, dry conditions. Cover the soil with foil to keep it in place while spraying the tree with the water bath. Apply the insecticidal soap spray after the tree leaves dry. Follow manufacturers instructions on the soap spray bottle.

More information is available from the University of California's Pest Management Program. You can read more about growing citrus indoors in our Guide.

Courtesy of NYBG Plant Information Service