milk skin, or something similar (see Molina)
iyxtzotzoliuhca. nata de leche, o de cosa semejante. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 37r. col. 2. Thanks to Joe Campbell for providing the transcription.