If you want to see Nella's exit, try to watch the episodes from spring or summer 1989. I don't know if it was Marland's choice, or if new producer Laurie Caso wanted to clean house, but that whole Pam/Nella/Beau group was written out after Meg's departure and with Paul/Andy/Lien/new arrival Duke taking up airtime. By Marland standards the writing for their exits is both aimless and downbeat - actually it's sort of a glimpse of what is coming with Marland's increasingly aimless writing. Beau and Pam drift apart, and Beau has sex with Nella, who has a thing for him. Pam leaves in summer to return to Seattle - oddly, just as her con artist father arrives in Oakdale.
Nella claims to still want to be a nurse, but misses a lot of shifts, to the point where I think her own mother (who was head of nursing by this point) basically tells her she clearly does not want to be a nurse and is just lying to herself. Nella decides to move to California with Beau. I tend to wonder if someone at P&G got cold feet about the interracial aspect (I think this was the first black/white interracial relationship on ATWT) because when they leave we're told they are just friends and not interested in making their relationship romantic.