Many Portuguese Jews (converso, marrano, or otherwise) took names of flowers or trees as surnames.
My knowledge of the Portuguese language is very severely limited, but it is my understanding that "amoreira" is Portuguese for mulberry; perhaps "Moreira" might have morphed from Amoreira.
[As an aside, here is a piece on how the Portuguese language diverged from Spanish:
Ken Ryesky, Petach Tikva, Israel kenneth.ryesky@...
RAISKY/REISKY, ARONOV, SHKOLNIK(OV), AEROV; Gomel, Belarus
GERTZIG, BRODSKY; Yelizavetgrad, Ukraine
BRODSKY, VASILESKY; Odessa, Ukraine
IZRAELSON, ARSHENOV; Yevpatoriya, Ukraine (Crimea)