These are called Scale.
The larger brownish ones are mature and will be fixed in position. If you were to scrape one off, you'd find a tough thin brown shell or scale that covers a soft body. Once they are mature like that, they are hard to control.
The young can look like young aphids from a distance. They move a little, and are often pale greenish to creamy color and can be wipe off the plants easily. They will lodge in any crevice, in any leaf axils, and under the leaves, on stems, anywhere they can suck on the softer plant tissue.
Control is often with horticultural or dormant oil, and with destruction of the most infested plants.