These are scale insects (specifically in the armored scale group, not the soft scale group). Scale feed by sucking up juices from within plant cells, and high populations can cause damage. (Low populations generally don't need intervention if a plant is growing well.)

These might be Japanese Maple Scale in particular, though it's hard to tell without more microscopic examination. Despite the name, that scale species can feed on a very wide array of plants. It has two generations per year in Maryland, though the next egg hatch won't occur until closer to mid-May or later.

Scale insects barely move once the hatchlings settle-down (which happens fairly soon after hatching), so scale present on a new plant purchase had to have been present at the time of purchase. That said, dead armored scale look almost indistinguishable from live armored scale, in part because they don't detach from the bark right away and the shell-like body covering doesn't deteriorate that fast. This means that the nursery might have already treated the plant, or didn't notice they were present.

Either way, it might be worth returning so you can start with a scale-free plant since they can be difficult to suppress without resorting to pesticide use. There is probably little risk of them spreading to new plants until the first generation of crawlers hatches, so you have a bit of time to think about it depending on the store's return policy timeframe. There is no guarantee a new, clean plant won't also contract scale at some point in the future, since these insects are fairly commonplace, but generally landscape plants under stress are those that contract pest insect populations first (or to the greatest extent). Fortunately, once established, Chokeberries are pretty adaptable to a range of growing conditions.

If you opt to keep the plant and treat it, try first (gently) scrubbing the scale off the bark and treating the bark with horticultural oil. It's probably too late to use dormant oil (a slightly more concentrated type of oil) unless we have another cool spell, but either formulation will help to smother the scale with fewer risks to beneficial insects, especially if you can time an application to coincide with the crawler emergence period. The pages linked above will provide more information about treatment options and how to use hort. oil.

Miri

You're welcome. Be careful with rubbing alcohol, since it could damage plant tissues (including by removing the protective wax layer on the surface of foliage, when it's out).

Miri