This is just a guess so take it like a grain of salt. You said 'I have been letting the soil dry out before I water it. The leaves are green with a few of the leaf tips turning brown.' It could be just you let the soil dry out too much, so water moreoften. The soil must be moist always but not soakingly wet. As for the leaf tips turning brown it could be a pH problem and lack of Magnesium. Maybe too acid of a soil. As it happened to me recently. 2 days ago I noticed my Trovita's leaf tips were turning yellow.I tested the soil and it read 4.2. So I got me a dolomite lime and mixed them with the soil hoping for the best. The dolomite lime has a Magnesium mixed in it so I am solving 2 problems at the same time.

Some growers use a chalk-dust mix to spray on the leaves to act as a kind of sunscreen. This looks like that to me. Especially since you could easily wipe it off. It isn't anything to worry about, it will wear/wash off over time.

Yes, the pot looks small and the plant looks underfed. You are giving it only about a quarter of its nutrient needs: Citrus Focus instructions say "Add 10ml of Citrus Focus in every litre of clean water, weekly. If leaves are pale or yellow, increase frequency of application."

Silva- My trees look just like yours lately... I am in central Florida so took soil samples to UF 8n January. We were in a drought then. Numbers were ok. Few leaf miners but that is no big deal. Then about May? the trees starting looking poor. Much rain. Over the summer we have had 2-6" almost daily. On August 20th we had the total amount of yearly rain already. Took another sample of soil in and times/fertilized. UF says the huge amount of rain is washing nutrients from the soil throughout our region. The trees are not really 'thirsty' so even when fertilizer is applied it is not taken up because ground is always wet. Then that days rain ...and the next... and so on... wash away the nutrients. Everyone zI know is having same problem. Scary because I don't want our wells contaminated with fertilizer salts.