Your picture shows Hemlock Woolly Adelgid, an insect pest of hemlocks. It can kill the tree if left untreated.
There are homeowner products to treat the tree yourself, or an arborist can be hired to treat the tree.The tree can be treated right now. Compare the costs of purchasing enough homeowner available chemical to treat a large tree( you must read the product label to determined this, and most products have their labels online); or what it would cost to hire a professional.
Here is an article with details of what to use for control- https://www.canr.msu.edu/uploads/resources/pdfs/hwa_tip_sheet.pdf
If you decide to hire an arborist or a tree service, be sure who you hire is a licensed pesticide applicator.
Arborists by zip code can be found here- www.treesaregood.org