These are called hibiscus scentless plant bugs (Niesthrea louisianica). https://www.marylandbiodiversity.com/viewSpecies.php?species=17821

They are a native insect and primarily a nuisance pest on Rose-of-Sharon and hibiscus plants. They are actually beneficial for controlling velvetleaf weeds. Here is an interesting article about them.
http://bugoftheweek.com/blog/2013/1/6/mallow-on-the-menu-for-the-scentless-plant-bug-niesthrea-lousianica

If they are a bother, you can wear gloves and pick them off by hand (drop them into a can of soapy water) or crush them. Here is more information about them: 
https://content.ces.ncsu.edu/hibiscus-scentless-plant-bug  While NC State says you can use pesticide to control them, we do not recommend this treatment as it is typically unnecessary. Rose-of-Sharon bloom abundantly so if you can hand pick them off to control, it will help protect any harmful pesticide travel to beneficial pollinator insects. 

Some Rose-of-Sharon cultivars can be weedy and invasive from seed spread, so if these pests are too much of a nuisance,  perhaps you could consider replacement of a native flowering tree or shrub that could benefit more native wildlife. Native plants come with their own pests and diseases so its not to say they are problem free, but often their benefits outweigh issues. 

Let us know if you have further questions or would like ideas for alternative plants. 

Emily