(to do sth. ) concealing one's true motives by adopting a false cover, acting in an insincere manner etc (as specified)
under the guise (the pretext) of.
Сосед был человек неважнецкий. Комнату занял нахраписто после войны под маркой того, что инвалид (Трифонов 1). This neighbor was not a very nice man. He had seized his own one-room apartment right after the war, and rather highhandedly, under the guise of being a disabled veteran (1a).
Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. С.И. Лубенская. 2004.