That looks like Scale, they're a sap sucker and can multiply a lot if not controlled. You can wipe them off the leaves but they can get into small nooks and crevices on the plant. They can be brought in by ants so check to see if any are around or tending the Scale.

There are natural predators of Scale so you can just wipe off what you see and not use any poisons that might kill off the predators. Some people spray with a mix of alcohol and dish detergent diluted down. Otherwise you can check around gardening centres for what products are available for Scale control in your area.