Yep. That is Brown Scale, or Brown Soft Scale, or Soft Brown Scale, a very common scale insect. The tan insects that are visible are the older ones, and the almost transparent insects are usually males, but they can just be immature females. All of the ones I can see in the pictures have already settled down and won't move on their own: they have settled in to suck the juices out of the leaves and stems. Eventually, as they age, the females will turn a dark brown color and be more rounded: that is about the time they give birth to little baby scale insects.

Not visible in your pictures, but possibly present, are the little tiny ones, which are still mobile. (If you only had a single breeding female scale on one tree to begin with, the scale insects you see may be the only ones). If the little crawlers are present, they will move to a spot they like, and then settle down and do what the adults do.

Normally, scale insects don't cause much of a problem when you put healthy citrus trees outside for the summer, since the outdoor environment is not conducive to their spreading, but if you put trees with an infestation outside you are possibly going to have a problem. Indoors, because of the dry air and steady temperatures, a few scale insects can very quickly become a serious problem. Left unchecked, they can weaken and eventually kill a tree.

My preferred method of control is two-fold if the plant is small enough and the infestation is not too overwhelming. First, I like to rub the little buggers out with my finger or a pencil point. In particular, I look for any particularly large and rounded scale, since these are the ones that are going to produce the nymphs (they may already be under the large adult).

Disturbing the adult insects doesn't take much pressure. It may not be necessary, but the insects have a waxy coating, and I think it probably makes it easier for the next step to work. I don't rub the scale insects before the second dose.

The second step: spray with a horticultural oil spray twice, at an interval of about 7 to 10 days. Another way to go would be to use something like Neem oil extract, but I have not tried that. The Horticultural Oil route is nice because it is easy to mix up a quart of the solution, spray the tree or trees, let them drip dry, and then bring them back inside. The brand I picked up at the local nursery has no odor and can be used up to the day of harvest. I'd say that most of them on the market are the same, so I wouldn't worry too much about that.

So, I would spray the trees twice, 7 to 10 days apart, and the problem should go away for the summer. Make sure you spray the tops and bottoms of all the leaves, and the branches and trunk. Make sure the trees aren't in need of water before you spray them. If they are, water them, and wait a day before spraying.

Then, in the fall, I would spray twice again shortly before they come back inside.

Even with this regime, it is still possible that they will show up again around December-March, since it really only takes one of them to start another infestation. I've sprayed some of my trees each winter (usually just once) at some point or other when I see the insects. This year they showed up on three of my trees. I just pick a warmish day (temperatures above freezing, sunny, no wind), spray them, and then put them on the unheated porch to drip dry. They are only outside for 10 to 15 minutes, tops. The goal in this case is not to eliminate all the insects, but to reduce their numbers enough that the problem is insignificant for the rest of the indoor season.