Aphids.
Put a small amount of dishwashing soap in a spray bottle (you don't need alot) and spray each leaf front AND back, that is where they lay their eggs, real good, then the branches or stems also.
Don't rinse it off. Let it dry on.
Wait a couple of weeks, if you still have a problem, do it again.
You won't have to do it twice, but you can use Dawn dish washing soap and water whenever you have a problem.
Good Luck!