This is biblical quotation. In it's full context, the phase is used to illustrate that the word of God is eternal and consistent compared to human life which is transient and brief.
In this context in the poem, this ties in with the religious imagery. It has already been suggested that the photographer is spreading an important message through his photos (as a priest spreads his messgae through the word of God). This quotation develops that highlighting how the suffering the photographer shoots is constantly changing and on-going because photos succeed in capturing it and making it permanent.
The rhyming of 'mass' and grass' as well as the mention of Phnom Penh might also invoke the mass graves or burial sites, where the bodies/victims of genocide/war decompose and literally turn to grass in the soil.
The juxaposition of this phrase with the list of which is perhaps an ironic comment: yes, all human life ends eventually, but in such places as these it ended much sooner.