These are sap sucking insects which will weaken a plant over time. They appear as hard round “bumps” on the underside of leaves and along the stems. They can also emit sticky honeydew, which can be followed by sooty mold.
These can be difficult to treat at the adult stage because they are covered by a hard waxy covering that is impermeable to sprays. If caught early, a cotton swab dipped in rubbing alcohol (50/50 alcohol/water) can be used to wipe them off. Horticultural oil may also be used to spray the plants.