Even though you've only given us just a smidgen of information, I think we can pretty much guess that your plant is probably infested with aphids. A bad infestation can certainly weaken your plant. Aphids will multiply out of control if the population is not managed in some way.
Aphids don't chew plant tissue, but suck juices from the plants. So, they are busy doing damage all of the time.
Spraying them with strong streams of water can be a help, but you need to do it often. Insecticidal soap applications can help, as can Neem oil. In the winter, you can apply horticultural oil to the stems.
In most cases, my option for aphids is the hose and my fingers. There are lots of predators and parasites of aphids, and the use of some of the other products may hinder their efforts. It's up to you to decide if the population on your plants warrant a little 'extra' help. I keep neem and commercial insecticidal soaps in my cupboard just in case.
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Japanese modern garden
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