This is scale or mealybug, they are insects that suck sap out of the plant. So yes they are harmful and it is better for your plant to get rid of them.
There are several ways to get rid of them (in my opinion there is no best way), the easiest is to rub them off the plant or spray with soap. Also poisons are available, but none of the methods are 100% effective (that is my experience with scale insects).
Here some instructions and more info.