DoDI 1325.06 addresses off-base demonstrations by service members. https://www.esd.whs.mil/Directives/issuances/dodi/.
MARADMIN 662/19 regarding engaging in political campaigns and activities. See https://www.marines.mil/News/Messages/Messages-Display/Article/2029669/guidance-on-political-campaigns-and-activity/.
ALMAR 008/17 provides guidance, generally, for Marines concerning unofficial online activity that has an adverse effect on good order and discipline within, or brings discredit upon, the armed forces. See https://www.marines.mil/News/Messages/Messages-Display/Article/1113849/social-media-guidance-unofficial-internet-posts/.
Violations of the above-referenced guidance may be prosecuted under the UCMJ to include, but not limited to, Articles 92 and 134. Additionally, Marines must recognize that the UCMJ also specifically prohibits Contempt toward officials (Article 88), Communicating threats (Article 115), Riot or breach of peace (Article 116), and Provoking speeches or gestures (Article 117). See https://jsc.defense.gov/Military-Law/Current-Publications-and-Updates/.