From the press release:
Branch, 37, tested positive for ipamorelin as the result of a urine sample he provided out-of-competition on May 24, 2019. Ipamorelin is in the class of Peptide Hormones, Growth Factors, Related Substances, and Mimetics and prohibited at all times under the UFC Anti-Doping Policy, which has adopted the World Anti-Doping Agency Prohibited List.
Ipamorelin is a potent Growth Hormone Secretagogue that stimulates the brain to release human growth hormone and is used by athletes as a performance-enhancing drug. The use of prohibited peptides by athletes and consumers for performance or physique enhancement purposes (including recovery from injury) poses serious health risks, and these peptides are not approved by the Food and Drug Administration for human use or consumption.