The headword is a present active participle in the feminine accusative singular. It is taken from one of the autobiographical poems of Gregory of Nazianzus (PG 37 1380.2), with a gloss -- also in ps.-Zonaras s.v. -- drawn from commentary ad loc.
A similar gloss appears in the scholia on Nicander, Theriaca 308 (where the lemma is mudo/wntes).
For the verb muda/w see also mu 1374, mu 1377.