Thank you for attaching the images. Your beech has aphids, a common pest of these trees. In some instances, the reverse of the leaves can be so heavily infested they appear to be covered with white felt.
You can help limit the aphid population by washing the aphids off the tree with a forceful water spray as high as possible with your hose; repeat every several days as needed. Any remaining aphids, or those born later, have many insect enemies common in every garden and landscape.
Or you can hire professional pest control to the spray the tree. (See “Selecting a pest control Company” at http://npic.orst.edu/pest/selectpco.html ). And, then, you can locate a nearby Certified Arborist for the task by searching with your zip code at https://www.treesaregood.org/findanarborist/findanarborist.