It looks like scale. For a small plant you can use a cotton swab or a sponge and alcohol and rub them off. Citrus likes well drained soil, but in a pot you will need to water it more often if it does not contain a lot of organic matter to hold on to water. I use citrus food vigoro 6-4-6 and osmocote for most of my potted plants. The package will tell you how often and how much to apply. Tree fertilizers are based on the diameter of the trunk so the amount varies depending on the age, stage of growth, and diameter.
When the new leaves come out citrus are often attacked by scale, aphids and whiteflies so you do have to look under the leaves every week. Put out ant bait at the base of the trees. I use outdoor terro. The bait stays in the container so it does not contaminate the soil. Ants need to be controlled or they will protect aphids, scale, and mealybugs from predators. Asian psyllids and citrus brown aphids can transmit fatal diseases to citrus like Huanglongbing and tristeza.
They look like seedlings. Citrus trees will take 5-7 years to fruit from seed unless they are grafted.