Maybe I'm conflating RM's situation with someone else's, but I seem to recall he had some sort of injury (in a car accident or something) that made it harder for him to remember his lines. His general frustration with the production team (likely the transition from the Brooke/Ridge merry-go-round to the Hope/Liam merry-go-round) combined with the memorization issues compelled him to leave. I think it was probably a step too far to expect them not to recast such a central role.
I also recall that the Head Writer of GH at that time, Ron Carlivati, trolled B&B onscreen about the Ronn Moss departure. Young Spencer Cassadine (at the time about nine years old) was trying to impress Emma Drake, so he claimed to her and half the town for several weeks that he had hired Ronn Moss and his old band, Player, to perform at the Nurses' Ball. Fans of GH were shaking their heads (a nine year-old loves 1970s soft-rock?) and of course the soap press were fit to be tied. RC made it appear that Moss might make the appearance to spite his ex-bosses at B&B, but in fact there was never any discussion with Moss or the band about appearing.