both refer to movie chains (i.e., Loews, AMC Theatres) with movie theatres that screen more than one film at a time, as opposed to single-screen theatres. A multiplex has from 2 up to 16 screens, a megaplex has 16 or more screens; plex is the abbreviation for a multiplex theatre.
A morula (Latin, morus: mulberry) is an early stage embryo consisting of cells (called blastomeres) in a solid ball contained within the zona pellucida.
While migrating down the woman's Fallopian tube to her uterus, it is named a morula, and from days five to 12 post-conception, a blastocyst. The blastocyst implants in the nutrient-rich lining of the mother's uterus.