It looks like it might be spider mites. When you examine the plants closely, do you see little strands of web? The presence of the strands of web would confirm that it's spider mites.
You can control them by spraying with a pure soap mixed with water. Be careful to read the label to be sure you have soap and not detergent. Cynthia suggested a mixture of 1 tsp of soap per quart of water.
Be careful to spray the undersides of the leaves, the stems, and the tops of the leaves. Repeat the application about every 7 to 10 days, to kill any new mites that hatch. Three to 4 treatments should have your plant clear of mites.