Looks like spider mites; you won't see them without a good magnifying glass. A good wash outside or in the shower will go a long way to getting rid of them.

My advice... lose the fertilizer spikes and replace with a good soluble citrus fertilizer applied 3-4 times per year according to the label. Your tree looks to be a little underfed; and unhealthy trees are more susceptible to spider mites. I would also get a good foliar fert. and give your tree an application of that about every 15 days; I like Bayer Bayfolan Forte; lots of container growers here use Foliage Pro... there are others.

Your tree looks a bit leggy; I am guessing it gets less light than it would like.