I agree, it would be a good idea to have an arborist look at your Arborvitae. I can't give you specific pesticide recommendations, and scale is difficult to deal with otherwise, especially if it is well-established. However, if it is a fairly new infestation, you might be able to treat it with these cultural methods, from the PNW Pest Handbook:

Scrape scale insects off plants by hand with fingernail or toothbrush.
Prune off major infestations in trees and shrubs if possible. Apply tape, sticky side out or use a similar adhesive near infestations of
adult scale to intercept and catch the crawler stage. Water landscape plants and trees properly, as drought-stressed plants are more
susceptible to scale insects. Avoid excessive nitrogen applications as this encourages growth in scale insect populations. Generally, pesticide
applications are applied to the vulnerable crawler stage before the scale covering is formed.

Here are links to more information:

https://pnwhandbooks.org/insect/hort/landscape/common/landscape-scale-insect

https://pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/arborvitae-thuja-scale-insect