FTM stands for "Female to Male," referring to a transgender individual who was assigned female at birth but now identifies as male. In the context of porn video tags, this term indicates that the performer or subject is a transgender man (FTM).
"Moan" in this case simply refers to the act of making vocal expressions of pleasure and excitement during sexual activity. So, "ftm-moan" means a scene involving a transgender male performer expressing their pleasure vocally, typically through moaning sounds. This tag is specifically for adult audiences experienced with porn videos