Justice March is a left-of-center advocacy organization and project of the Sixteen Thirty Fund, an advocacy nonprofit managed by the for-profit consulting firm Arabella Advisors. Unlike many projects of the Sixteen Thirty Fund, Justice March does not appear to have an online presence or any traceable list of staffing. 
Justice March is a registered trade name of the Sixteen Thirty Fund, a left-wing “secret-money” advocacy group that was active promoting Democratic candidates during the 2018 midterm elections.  Sixteen Thirty Fund registered the trade name Justice March on April 12, 2018 with the District of Columbia Department of Consumer and Regulatory Affairs.  The trade name expires on April 11, 2020. 
Many of the Sixteen Thirty Fund trade names were for groups very active at the local and state level on behalf of Democratic congressional campaigns during the 2018 midterm elections–usually criticizing Republican candidates through issue advocacy without direct endorsements. Projects had individual websites, Facebook pages, aired commercials, and promoted volunteer efforts. Many listed staff and executives. 
Justice March does not appear to be associated with a specific event or demonstration.
Secrecy of Sixteen Thirty
Politico has called the Sixteen Thirty Fund, a 501(c)(4) advocacy group, a “massive ‘dark money’ group.” The news outlet also said the Fund has “contributed to efforts ranging from fighting Supreme Court Justice Brett Kavanaugh and other judges and nominees named by President Donald Trump to boosting ballot measures raising the minimum wage and changing laws on voting and redistricting in numerous states.” 
The Washington Post editorial board in November 2019 called the Sixteen Thirty Fund a “big campaign donor,” and noted the scale of large contributions (among them a $26.7 million donation to the Fund).  The $26.7 million donation likely originated with the New Venture Fund, the largest of Arabella Advisors’ in-house nonprofits.