The headword is the present participle, masculine/neuter genitive singular, of muka/omai. It has the same glossing in the Lexica Segueriana and one would therefore think it quoted from somewhere; note, however, that other lexica (see the references at Photius mu587 Theodoridis) have masculine accusative plurals -- with mukwme/nous likely to be quoted from Plato, Republic 396B (of bulls).
cf. mu 580, mu 1392, mu 1394.