These are the nymphs (juvenile life stage) of the Spotted Lanternfly, an invasive insect that has become numerous in several Maryland counties. You can learn more on the linked page. The tree they are covering is Tree-of-Heaven (botanical name Ailanthus), also an invasive species. We recommend its removal whenever possible. Information about removal options are included on the linked page, since just cutting it down will not be enough; generally, herbicide is the best approach before cutting the tree down (after the herbicide treatment has had time to work).

Miri