The actor's fiancée, Paige Press, tells us Musser passed away Friday night at his home in Del Mar, CA. His uncle, Robert, confirmed his nephew's death to us as well ... but neither he nor Press would elaborate on the circumstances or exact cause.

Musser will best be remembered for playing Del Henry on the beloved, long-running soap opera between 2005 and 2007 ... during which he appeared in a total of 43 episodes.

Other films/TV shows he starred in include ... 'Grown Ups,' 'Rita Rocks,' 'Desperate Housewives' and 'Road to the Altar.' He only had 6 acting credits -- but Musser was known in showbiz outside of his on-camera work ... namely, through is fitness modeling career.

He had graced the covers of several magazines -- including Men's Health, Men's Workout, Exercise Health and others. The guy was a bodybuilder, and had a great physique.

AM might not have acted in H'wood lately, but he was still a big presence on social media -- highlighting his continued health/fitness journey, his active lifestyle and even pics with loved ones. On its face, nothing seemed to be amiss ... making his passing all the more shocking.

Musser was only 50 years old.