Calamondin Tree Has Brown Spots
Charles County Maryland
The information on this page will help with overall care for citrus, particularly when grown indoors in our area: https://extension.umd.edu/hgic/topics/growing-dwarf-citrus
The cause of the plant's leaf damage is unclear (it looks a bit like residue, so maybe you can rub/rinse it off), but its overall color suggests that it is nutrient deficient. While normally you would cease fertilizing houseplants now (or soon) as they prepare to slow growth and "rest" during winter, since this plant's deficiency is a health issue, we suggest giving it at least once dose now. (See the package label for instructions on how much to use and how often to re-apply. Once it regains a normal deep green color, you can stop fertilizing until spring.) Bear in mind that fertilizers take time to work and affect foliage, and that some nutrients cannot be replenished in old foliage and instead will only result in new growth developing with a normal color. In time the plant may shed these older, discolored leaves, which will be normal.
Other than supplementing its nourishment, make sure it's getting enough light, especially when it comes back inside this fall, and is being allowed to dry out some between waterings - citrus do not thrive when kept constantly damp. (Overly-wet roots can eventually fail to function properly, leading to nutrient deficiencies. If you use a saucer to collect pot drippings, make sure it's emptied as soon as you're done watering so it doesn't keep the soil too wet.)