I was informed by a knowledgeable viewer: The figure wearing a mask is identified as Theonoe, and next to her is a nurse (Trophos). The text on the altar says, "Kointos Kalpournios Eutuchês epoiei", that is, "Quintus Calpurnius Eutyches made [it]". This refers to the name of the drama the mosaic represents (cf. Menander's plays Perikeiromene and Theophoroumene). Theonoe
was a priestess, a character n Euripides' play Helen, and a figure in a complex comic plot, to which the mosaic here may refer; see myth #190 at: a site specifically for these stories.