Fans gather at a Feastables launch event in Sydney, Australia.Don Arnold/WireImageInternal data shows growth at MrBeast's candy company, Feastables, has slowed.The brand has been a cornerstone of the top YouTuber's expansion plans.Feastables installed a new leader this year and is betting on additional products.MrBeast's sugar rush is fading.Growth at the top YouTuber's candy company, Feastables, has slowed considerably in the last year, accordi…